I agree with Manbearpignick and Adam. I think educating oneself is critical, especially when one can drop thousands on modules and then discover they're not the right ones. Know the sound you're trying to get, then learn how to get that sound, rather than just buying whatever modules you see someone else is using (not that you're doing that ) and then hoping it'll all come together (which it very well might).
+1 on the ES-9 and using VCV. I think VCV is probably the single most valuable educational tool there is right now.
To answer your VCA question in a word: yes. "You can never have enough VCAs" is a modular synth catch-phrase for a reason. You will use VCAs for literally controlling everything. You will also need a good way to get audio out, and a VCA can certainly be used for that. The Tallin is one example of a good multi-purpose VCA/mixer. And, as Adam pointed out, I'd recommend limiting your output to mono and save the capital spent on stereo circuitry for other bits.
I always go back to the basics whenever I find myself asking a lot of questions about something, no matter what it is. With modular, that means going back to the tried-and-true building blocks of all (subtractive) synthesis: at least one VCO, then a VCF, EG and VCA(s), more or less in that order. Repeat that group again. To get the techno you're after, I'd recommend at least two VCOs (I only see one, the Mantis). I suppose you could use the Disting as a VCO, but then you'd be sacrificing all the effects that critter has under the hood (watch out for running too hot a signal into the Disting...it clips a bit too easily for my tastes and can sound a bit crunchy).
Look at Erica Synths Pico modules for drums. Good stuff there.
The techo tweak happens from time to time by running your signals fat and then sweeping them into a tight little spiral using a good filter. By "good" I mean that you don't want to lose fidelity just to sweep something into a high-pass shape. I have found that the Xaoc Belgrad is a great filter with a huge number of shaping possibilities that also has excellent signal integrity. The Polaris, which you have, is an excellent filter also. Patch any of that that back into your VCOs to get some really interesting FM possibilities.
Maybe the overarching point here is that the sound you're probably after could be created using a far simpler set of modules than what I see in this rack. You def need to start with a sequencer of some kind and/or a beat machine. The rest is just riffing monophonic notes from several VCOs and tweaking the juice out of them with some wide ranging filters. Keep in mind that the Polaris also has some cool phasing ability, so if you used an envelope follower to bring that back into a (aforementioned missing) second VCO, you could get so sick you'd end up in the hospital with a gaggle of goth babes losing their minds in the waiting room.
"I'll just plug this in here and see what happens."