Choosing the midi interface and the “utility” modules is the next step to do after you chose the cabinet.
First the midi interface is important because it behaves as a connection between the digital world (imidi, computer/seq etcc) and the analog controls.
If you want a great amount of midi motions on the parameters you have to choose a good interface with many cv like the Analog Systems RS-140 ,  if you are more “analog inside” and your point of view is just to send notes and just some basic control , a cheap and small Doepfer A190-2 is perfect.

I chose the Doepfer for two reasons, first because I want a small system and the Doepfer is really small, second I ‘m not a fan of midi controls over CV because they are two different worlds that speaks different langfuages and it’s difficult to get the result you want with a midi controller, it’s easier to twist your knobs with your hands or an analog sequencer.

The third possibility it’s not to buy a midi-cv module and use a DC coupled audio interface with the Motu Volta software.
All the MOTU  audio interface from a couple of years are DC coupled , so it sure run for example on a MOTU 828MK3, I don’t know about other manufactures’ products.

Another necessary module is the multiply.
A multiply receives a cv/gate signal and outputs the same signal on more outs, so you can send a keytracking cv to control the pitch of two VCO and control the tracking of the filter…
One source to different destinations at the same time=Multiply!

Attenuators are other utility modules.
On many modules the cv  in jack has its attenuator (=amount), but the more esoteric stuff does not have attenuators…
another case iwhere attenuators are necessary is when using modules from different manufatures that interact together and they use “different cv standards”, the attenuator is used to get the signals at the right level.
A mixer or a VCA can be used as attenuator because they can reduce the amount of signal output.