Some advice for some modules to start?
OK...a basic rundown of the types of modules is useful here...
GENERATORS: oscillators and pretty much anything else that can make a noise
MODULATORS: sources of control signals which alter the behavior of other types
MODIFIERS: modules that alter signals from the above two types. Filters, effects, etc. And...
CONTROLLERS: the stuff you use to interface with the first three.
If you're missing one of these, you don't have a synthesizer. Also, it's worth noting that some modules do a few of these functions at the same time, such as what you get with performance mixers which fit into both modifiers AND controllers. It's important to keep an eye out for these, as you can get more function into a build by using these "Swiss Army Knife" modules. Case in point is one of my fave small rig VCOs: Klavis' Twin Waves mkii. That module not only has two oscillators, but the control inputs are quantized, so you've got both "generators" and "modifiers" there. At the same time, DON'T get modules that either induce eyestrain and/or require tweezers to safely tweak settings. Those are no fun whatsoever, so make sure your build's ergonomics are in a zone you feel comfortable with. Basically, you need to ask "will I feel comfortable using this in five years?" as sort of a sorting guideline.
Second, before diving further into the $$$ zone with hardware, start your inquiries in SOFTWARE. And when you say "software" and "modular synth", you may as well just go ahead and say "VCV Rack". It's probably one of the best ways to sort out what YOU want modular to do, because VCV is a very good replication of the Eurorack environment...some modules are even software versions of the hardware ones you'll find here. And the big selling-point here is that...it's NOT for "sale", it's FREE. https://vcvrack.com/ It's a much better way to learn while doing, because doing so with hardware involves a good chunk of cash. The sole caveat about VCV Rack is that it does tend to be something of a "resource hog", so in my case, I've actually gone with a wholly separate machine for it to chew on. Not a bad idea, really, especially since some tiny machines (mine's a Lenovo ThinkCentre, Gen9 i7-8700 6-core, 32 gb RAM) are both beefy AND cheap, which makes them VERY useful for that purpose.
And third: when you go with hardware, do not neglect utility modules! I can't stress that enough; a module with a couple of attenuverters might seem boring, but when you need to invert a signal, you'll be very glad that those attenuverters are there...boring or not! Jim and I are in 100% agreement here...ALWAYS make sure you've got utility modules that expand the capabilities or, in some cases such as VCAs, are 100% essential. And this is also part of the reasoning behind starting with a bigger cab, because those "sexy" modules require signals that make them do what they do, and you'll need space for those. F'rinstance, take something like Intellijel's Morgasmatron, which is a complex dual state-variable filter. So, filters usually require envelopes...so you're also talking about a pair of envelope generators there. Then you'll probably want a pair of VCAs on the outputs. And with LFOs to provide tremolo/vibrato, you wind up talking about maybe a total of 30-40 hp for that Morgasmatron and not simply the 20 hp module itself. That sort of thing. I'd suggest looking at some of the "classics", such as the venerable ARP 2600 (probably the best teaching synth ever) to see how their designers implemented these things to create synths that are still on sale 50 years later. And speaking of 2600s...
DO take advantage of some of the prebuilds out there. Uli's 2600 clone (which actually has ALL of the ARP 2600's v.3 and on functionality as it retains the 3620 submodules, while Korg's "M" reissue is missing that) only runs about $700...and as a longtime ARP 2600 user myself, I can safely say that Behringer won the "version 5" competition, while Korg's stumbled here. But ARP 2600s (irrespective of WHO made them as long as they're functionally complete) might just be one of the best "core synths" for a larger modular setup, and something I can recommend equally for any and everyone...especially starting in modular, because it's easy to use and make sense of, and you can patch loads of other devices into/out of/through it since it uses the Eurorack standard of 1 V/8va and positive gate/triggers. I would call that $700 cost a bargain!