I am looking to put together a modular trying to chase 3 things:
1) Some basic Moog type sounds (I know not exciting, but I don't have a fixed architecture Moog-ish mono synth)
2) Some juicy Roland filter type action for Juno / Jupiter leaning lead sounds as well.
3) Some ambient / textural string type sounds.

I am thinking the following to start:
- Mother 32 as a base. It has great saw wave and VCF (to my ears). I have tried but can't figure out a VCO, VCA, VCF combo for as cheap as I can pick up a Mother 32 so I keep coming back to that. The sequencer and basic LFO are just a bonus, plus I get the MIDI to CV without spending money on a separate converter.
- Studio Electronics 8106. Adds some variety and juicy Roland type filter as an alternative to the ladder filter of the Mother 32.
- A basic ADSR since the Mother 32 lacks it, and I really like a good bit of release in string type and lead sounds. The Noise Reap Dopes unit is cheap and comes in a visually cool black, with the ability to be dual AD envelope generators which = versatility.
- Mutable Rings. Self-explanatory, but mainly to be a processor / sound mangler for drawing out string like resonance from otherwise normal synth patches. I'm definitely sold on this as an important part of the rack.
- Mutable Tides. I'm on the fence about this one. Thinking maybe it could help to get longer evolving textural sounds by using the looping, and it seems to cover multiple things. The wave folder seems to hold interesting potential.
- I think I will need another basic VCO, something with at least triangle and saw, since stacked saw waves for detuned sounds are cool, and also since the Mother 32 lacks triangle altogether. The Studio Electronics Slim-O looks like a possibility but at $200, I know there have to be other options that sound as good or bring something interesting.
- I will probably want to add a more elaborate LFO later.
What else am I missing? Is there any module I should consider that maybe covers multiple bases for ultimately versatility??

I will be mainly playing via a MIDI keyboard and sequencing in my DAW both. I have effects like stereo delay and reverb covered well with my existing guitar pedal board. I have a homemade 84HP case, with the ability to expand it to dual 84HP by buying more Z rails, so I am limited by money overall more than thinking of myself as limited by case size.

Done...and I think I hit all three targets:
ModularGrid Rack
Moog: obvious.

Roland-ish synth part: OK...in this case, I went with two Klavis Dual VCOs, because to get that sound right, you need some oscillator detuning and, potentially, sources for sync and/or suboctaves. Filter is a G-Storm JP-6 clone...doesn't get much better than that for the Roland analog polysynth sound in modular format, I think. Then it gets interesting; the mixer allows you to do some inversion of VCO waveforms, which can also cause some nice phasing results, done right. The Klavises, also, have the plus of being internally-quantized, so running them right off of the M32's sequencer is easy. Two EG + VCAs, two VCS...not a bad modulation section. But the last triple LFO, that's specifically for the triple bandpass filter...and this gives you that counter-swept-ish resonating behavior common to some really great divide-downs such as the Polymoog, Korg PS-series, et al. Closed that up with a delay, because...well, a delay. Just puts the cherry on that late 70s-type string synth process w/o wasting an external box.

Rings is still there, natch. Then there's a 3x VCA for CV-controlled mixing to mono, and a metered Ladik mono out.

Note that I swapped the P/S...like I often say, overspec that part. It puts less load on the P/S, reduces heat and component strain, and makes for a more reliable build. Added a mult, too...necessary if you're going to do your MIDI-CV conversion in the M32, then send that upwards to the upper row's module compliment.

Not too shabby...yes, a bit spendy in some areas, but I managed to avoid some areas you were heading toward that might've spelled some sizable expenditures while still coming up with major capability.

Cool, thanks!

The Jupiter filter is a little spendy, even compared to the SE 8106 which seems to get some rave reviews. I've never played with hardware Jupiters to care about total accuracy. Will have to try and find some demos online to see if it sounds good enough to sway me.
I had not really thought about syncing VCOs since I often didn't bother with it in hardware synths I've had. Guess I assumed I would just stack a dissimilar saw wave VCO with the Mother's saw wave and patch thru a different filter for Roland-ish tones. The Klavis VCOs seem pretty deep, likely deeper than I was aiming for. What about Roland's System 500 dual VCO? That would be more affordable than two Klavis and they have sync capability. Most of the demos I have found sounded like farting sound noise, not very musical, so it was hard to gauge it's potential.
I like that triple LFO, seems a lot of versatility in small space.
You're talking over my head a bit about the triple band pass filter and modulating it with LFO. Am I correct to think of that sutble (or sometimes not subtle) whooshing or sweeping sound I sometimes hear in a lot of OBX and Jupiter poly sounds? If so that is intriguing. I was thinking this would stay a monosynth only modular setup but I could likely use that sound even in some lead and fx patches for sure.
Is the multi module necessary to send CV pitch to multiple devices from the Mother?

My own JP-6 is sitting to my left, about six feet away. Trust me, you want accuracy here. The VCF arrangement in the actual synth is utterly amazing, not exactly like anything else Roland did before or since. It has its own particular sound, but it's very chameleon-like...it's capable of going in a lot of other directions than the obvious.

As for the Klavis Dual VCOs...note that. They're dual VCOs...each module contains two VCOs, plus quantizing and several other tricks. So what you see there isn't two VCOs, it's two modules that contain four VCOs. The Roland/Malekko stuff can't get you in that ballpark for Klavis's price, believe me. Demos, though...look, modular is very open-ended. If someone does a shit demo video, it doesn't necessarily mean that that's going to be your result when you program it alongside your modules. Anyway, yeah...four VCOs, which means you can do a lot of different potential directions, such as two-voice paraphonic, using all four in a stack, or using some elaborate crossmod or sync schemes.

At the same time, however, modular is, by default, 'deep'. And, annoyingly, trendy. That's a bad combo; you have people thinking that 'wow...these modules will solve everything in my music!', and that's NEVER the case. Consequently, I keep hearing of people diving into this, making a lot of wrong assumptions, getting hosed on money, and still wondering why their music isn't clicking...and the fact is that they could've saved a lot of money by just looking in a mirror to find the problem instead of dropping several large and discovering it the hard way.

Going into modular synth work isn't a casual decision. It requires a sizable amount of background information, research, and outright scutwork to sort out whether or not this really is a viable direction (for starters) and then, if so, what next? My advice: the rack above will work, but if the concepts seem somewhat beyond your comfort area, step back before massive cash gets thrown around and dig a lot deeper into this first. Figure out why things sound the way they do, both on recordings you're familiar with, instruments you're familiar with, and relate that to the modules you're not familiar with. Do get the M32, maybe another patchable or so first before a headlong, thousands-of-dollars dive into the deeper end of the pool. You'll likely be a lot happier and more satisfied in the long run.

What has steered me toward even considering modular in the first place is that while I love the sound of various hardware synths, they all seem to paint you into a corner. I liked my Minibrute for what it was but grew to want a different filter and snappier envelopes, which made me think I would be happier with a Moog Little Phatty or Sub Phatty. After demo'ing a Sub Phatty I realized it scratched an itch, but it would limit me in it's own way too in short time.
I'm always drenching synth sounds in delay and reverb or looking for other means to try and create layered ambient sounds, so when I heard some of the stuff people were doing with Mutable Instruments modules that can't be done at all on existing fixed architecture synths - then it felt right.
I might scale back my ambitions, get the Mother 32, an extra VCO, and the Rings - then think harder about what's actually missing. I'll likely need time for my wallet to catch up anyway.

From the prebuilt synths you're describing, I think it might be possible that you're painting yourself into a corner by limiting the budget on these. The M32, small Phattys, original Minibrute...all of these are pretty limited solely by the fact that they're inexpensive and therefore don't contain the complexity of higher-end prebuilts. But when you start getting into the more complex monosynths, or going away from the analog (which is another 'too trendy for its own good' thing) end of synths to explore good digital synths, you get back into the sort of sonic complexity I think you're aiming toward.

Again...back up and do some research. The solution to doing things over long-term is to never act quickly, but consider where you want to go in the long-term. And this takes time. Hell, you might find you can get sounds out of an old Casio CZ-101 that are just what you've been thinking of, but without a lot of research into where you should go with your musical ideas, you'll never figure an intuitive leap like that out.