Basically, what you need is simply a single module: an input preamp. At least, if all you plan to use the modular for is processing. If you want to control the modular with the M1, then you'll need a MIDI-CV conversion module as well. As for whatever goes in between those, that'll depend on what you're trying to do with the build overall.
However, one thing I'd also like to point out is that if you have all of these synths and drum machines, you really should consider treating THEM as "modules" as well, routing all of their I/Os to a central patchbay. I've used this working method for over 25 years now, and it massively speeds up the interconnection process between devices.