With an octotrack, you'll need a midi to cv converter. There's some cheap single channel options, nice complex ones with their own sequencing like Mutable Instruments Yarns. You could also get an Arturua Beatstep Pro which which convert midi but also sequence modular gear on its own. For less $ that modular midi-cv converters.
Doepfer? They have some good stuff, but the Basic System is mostly good if you want to do very basic, subtractive synthesis patching. And depending on where you live, not exactly the best deal. In Europe, Doepfer is much cheaper, but in the U.S. isn't that 'competitive'. Much of it is super simple and requires many more modules to cover what some more modern designs (like intellijel and make noise) can accomplish. ie, to cover complex osc territory like a dpo, you're going to need 2x a-111, and pair of vcas, a wavefolder, etc. Much more expensive and more HPs. I started with a lot of Doepfer, but sold it once I knew what I wanted.