Clouds or maybe Rings would likely make the most sense if you wanted to use a Mutable module to get started with this, as they have the most feature rich/fastest processor and most flexible I/O of the currently* released and open sourced modules.
Tom Whitwell also wrote a pretty nice article about how one could get up and running using Clouds as an evaluation platform:
*Note this presumably won’t be the case in a few months, though this also could mean some people may be inclined to sell their Clouds to replace it with the latest and greatest at that point.
An alternative route is just getting started with an STM32 Discovery board, which would be a much cheaper option if you’re willing to sacrifice out of the box eurorack interoperability. Incidentally, Tom Erbe apparently uses these in a class he has taught on basic audio dsp, and has shared a few resources here: http://tre.ucsd.edu/wordpress/?cat=21 Haven’t really looked in to them myself, but one of the example projects is apparently some sort of stereo, 3 voice, waveform-selectable oscillator.
Another thing to consider is that you don’t necessarily need hardware to start writing audio processing code that could some day run on ARM! Olivier has stated a few times how he likes to work with new ideas in pd (which you could write your own externals for), or writing code that outputs WAV files, examples of which can be seen in his “test” make targets, maybe Elements or Rings if you’re looking to write floating point code? https://github.com/pichenettes/eurorack/tree/master/elements/test
VCVRack could be a great place to get started too, it really does have a nice, clean interface, and would get you thinking more in a real time/“modular” context. Plus the example plugin looks about exactly like what you were interested in starting with (single sine VCO): https://github.com/VCVRack/Template