Thanks for the responses everyone – learning a lot.
The point about utilities being the secret sauce of modular is interesting. I guess, like Nick said, its hard to grasp at first because unlike oscillators, samplers, filters, envelopes, LFOs etc, 'utilities' don't really exist in the conventional world of music and are unique to modular (to my limited knowledge).
I'm pretty committed to 2 x 84 HP, just because I don't have a lot of space, and want to keep the arrangement of gear on the desk relatively flexible. Also I don't plan on building a massive collection of modules, I'll probably buy/sell stuff until I have a setup I'm happy with, but who knows... If I do eventually want to go bigger I'll probably just get another 2 x 84. Also I think I want to stick with eurorack because I'm not too interested in building a conventional synth – I like the range of unique stuff in eurorack.
I live in Australia which has unbalanced mains power. How much of a factor is noise? I've read that modules tend to put out a pretty hot signal, but yeah I might just follow Jim's advice and try without, then attenuate, then balance. I'm only running it into either an octatrack or external soundcard.
Re VCAs: what's the difference between a VCA and an envelope/LFO module? also VCA vs attenuator?
Thanks again, this thread is super helpful. Looking forward to placing my first order in a few weeks.
Here's the updated sketch. Trying to create a bit more space, now I can put one piece of cardboard neatly over the whole right side :) I might swap the listen four for PGH outs (handy for octatrack – and I think I'll only be working on one sound at a time with the modular, to begin with anyway). Also added midi in so I can use the octratrack as master clock. 