do you need vcas? probably, definitely if you want to form 'notes' instead of just drones - they are also useful for modulation - modulate your modulation!
do you need effects? not necessarily... but, most people find that at least one filter per voice is a really good idea... & if you don't have effects (at least delay & reverb) somewhere else (DAW, pedals) then you'll probably want some in the rack - they are useful in the rack as you can use them in other places in the chain than just end of chain processing... if you have pedals or line level effects then you probably want some sort of send/return (with level adjustment) to get them to work properly
do you need a mixer? again not necessarily - they are incredibly useful though - & you probably want multiple types - sub-mixers, matrix mixers (are always useful) and an end of chain mixer if you aren't going into an external audio interface or mixer - although you may want one even if you are - nb some vcas are also mixers - buy one of these (a dc-coupled quad cascading vca)
as a sub-mixer at the very least
what would I do? add utility modules... mixers, vcas, logic, sequential switches etc etc
see my signature for some handy hints on how to get the most versatility in your modular for the least cash!!!
hope this helps... & feel free to ask more questions!
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities