The Tex-Mix system is definitely worthwhile, to be sure. I'll second that, plus add some points to this...
When you're choosing a final mixer for a modular system, it's definitely a plus to have a VCA per strip, as this can take some load off of other VCAs in the system to use in more "interesting" ways. While you can get away with manual panning, AUX send/return and the like, it's super-useful to have CV control over levels at the mixer itself. For one thing, you won't need to add VCAs before it, which should save a bit of space. Plus, if you find a suitable mixer that can do CV over the other functions, those then eliminate OTHER modules from the build, such as panners, AUX mixers/distros, and the like. However, keep in mind that when you add modulation destinations, you're ALSO adding a need for more modulation. But there's "workarounds" for that that won't have you adding loads of modulation modules...just depends on how creative you get with patching and module utilization.
But as for output modules...my fascination with them has to do with many years of gigging with dicey AC power systems plus external noise sources (RF especially). See, in North America (and a few other places, like Japan), we've got unbalanced power...one leg of a three-wire AC connection here is "hot", one's "ground", and the last is "neutral", intended as a return for the "hot" line which has the entire 120-ish volts on it. Plus, you often see older construction where clubs are with bogus grounds...looks like three wire, but the electrician violated the hell out of codes by tying the "neutral" and "ground" together...a BIG no-no! Or maybe the stage has its power coming off of two different breakers, so you plug the mixer into one and, inadvertantly, plug the synth into the other...resulting in the lovely hummmmmmmm of a ground loop. Now, much of the rest of the world uses BALANCED power...there's no "neutral", but you get two 120V "hots" referenced to ground for balanced 240V. This is the CORRECT way to deal with AC...but remember, 'Murrika invent 'lektricity, so we am rite!
BUT...if you put in an isolated output, what'll happen there is that the audio gets through the transformer, but the noise gets chucked out, especially if it's trying to crawl back up your audio out line (like in a ground loop or induced RF situations). It's not NECESSARY, mind you...but it does serve as a good "prophylaxis" against garbage getting into the signal, especially if a user plans to gig with their system (or, more likely, gets called to gig with it unexpectedly) and there's questions about the power at the venue. If one of these is in the build, one just proceeds as per normal; if not, you might spend quite a bit of time that you don't really have chasing noise, and being forced to use a kludge to get the gig done, such as using a ground lift (which isn't a good idea!) on some of the gear. The easier thing is to sacrifice 4 or 6 hp for an isolated output module...then you KNOW you're set before problems ever arise.