The Monome modules can be added to that, although note that the case is already full with what it comes with (I think) - so you’re already into needing a bigger case if you want to add things.
You definitely don’t need to go overboard, but having a little space for future acquisition is a smart idea.
That Doepfer has a midi to CV converter module, so yes, you could use a MIDI-CV with it. There are lots of MIDI-CV options, though.
The usual question around building a modular is: what do you want it to do? Are you interested in building a more configurable monosynth? Do you want it to be a stand-alone instrument? What sort of sounds are you interested in? What sort of processes might you be interested in?
Enumerating what’s in the Doepfer:
- 2 VCOs
- an exponential VCA
- an ADSR
- a ring modulator
- a noise / random CV source
- a dual LFO
- a mixer
- a filter
- a mult
That’s a good list of the kind of archetypes you want - some sound sources, some sound shapers (filter/ringmod), some random, and then tools to modulate it with - but you might not necessarily want two analogue VCOs, or you might want a different sort of filter, or you might not care for ADSRs and just want some ADs. And I’d definitely add simple attenuation to the list of things you might want
Which again, probably all sounds complex, but if you don’t necessarily have a brief, your brief often turns into MOAR MODULES!!1 and frequently, “a simple deconstructed monosynth” isn’t always the start point people want.
I know you’re interested in the Monome modules, but they’re primarily compositional or control tools - they generate voltages and triggers - and you’re going to have to choose to have some sound-making devices, too. The Doepfer rig gives you one set of sound-making devices, and it’s definitely a reasonable choice, but I guess the one thing you’ve not mentioned so far is the sort of sounds you’re interested in.
Another place to perhaps consider starting is the Pittsburgh Synthesizer Box or a maybe even Moog Mother-32: both are single-voice Eurorack monosynths, that are semi-modular (they have CV controls for most elements, but they’re also normalled internally). With the Synthesizer Box, you can use it as a single voice, or use its components in another synth - feed something into its LPG, or use its LFO elsewhere. It’s not a module you’d keep, but it’s everything in one place to start. It all depends how confident you are with synthesis. (You could add a random source to it, some attenuation, and a MIDI interface and you’d have an interesting little starting set up to make noise with and explore patching). The Moog actually has a built-in MIDI-CV interface.
So you know, there’s an answer for everything, but it’s definitely worth thinking about the sort of tools you want to hand.