My recommendation in addition to what has been provided is to get a copy of the excellent book Patch and Tweak by Chris Meyer. It really is such a great book and will save you a lot of time, money and frustration learning modular concepts and how to put a system together. Chris also has a website of tutorials called LearningModular that I recommend. I use VCV Rack still to try out routings and modules that I don't have to buy until I try them first. It is free and very useful way to learn the basics of modular for FREE.
Now regarding Elektron connectivity and sequencer, I use the CV.OCD box that works great for sequencer to modular and using the effects from Elektron Octatrack to modular system. I have the Doepfer A190 but that thing was a royal pain to use. The CV.OCD is small easy to use and works with any MIDI controller/sequencer so it is great way to use external MIDI gear with modular. That or you could get an Expert Sleeper ES-9 module.