200e

I've been thinking  along those lines too. Although I'm thinking  for
budget reasons one module at a time and not stopping at 4.

As for myself, I'm weighing what I already have in non-Buchla modules
 .   

To give you suggestions I at least would think it's crucial to know if
you have other modular gear (and want to use it. And then what kind of
sounds you are after. Maybe you want to focus on processing external
sounds, maybe you just want to create a playable "monosynth", maybe
you are into drones and crossmodulation so you'd want a pair or more
of 261es (+259es if you are lucky).

Anyway I'd guess the most basic and generally useful combo would be 
261e, 291e, 281e, 292e. 

You might find the benefits of the 225e outweigh the 291e

Nick, 
who 's used a  200e system, but doesn't own anything