Maybe look at Cosmix vs that larger SUM2? 28HP vs 16
Starlab is a really cool looking module but not sure its 28HP in a smaller build cool :)
FX Aid Pro is 14HP (vs 28HP)...those two things alone will give you a lot of options in the future.
I have a two 6U Rackbrutes setup and my largest modules are Maths and Morphagene at 20HP and all my drums are external with MPC Live 2 and TR-8S...except for BIA which is in my rack
The only real way to tell if you're going to need more VCA's is to start patching things up and if you find yourself saying "damn, i wish i had another VCA" well...then you know if you need more VCA's :)
JB