+1 for everything above. Start with a bigger case like a Mantis, and get a copy of VCVRack.
For modules, you might also want to look at Grayscale's 18hp Permutation module. It does a lot of the stuff that the Turing Machine and expanders do and uses a lot less hp. Other good ones to look at for this kind of music are Ornament+Crime, Disting, Ultra Random Analog, Control Forge, Cloud Terrarium, Triple Sloths, and 4ms SWN and SMR.
Keep in mind that all this good advice you're getting is pretty expensive when you start buying everything. Starting with four modules is a good idea. I'd say get yourself a complex oscillator like DPO, Troika, or Furthrrrr Generator, a function generator like Maths, Grand Terminal, Zadar, or Scion, an irregular modulation source like Ultra Random Analog, Permutation, or Ornament+Crime, and an output module, like XOH or Intellijel's Outs. Will that give you everything you need to make generative music? Not really, but it will get you started. You'll be able to make sound, modulate it, modulate the modulation, and listen to it. Then get to know all four modules, figure out what they're not doing for you, and then go look for new modules.