Maybe I have to learn the Minibrute in depth before I even start building the rack?
Yes, yes, a thousand times yes. :)
The more you can get out of the MiniBrute (as far as knowledge), the better shape you'll be in when selecting and incorporating external modules. You can also think of it this way as well: the money you'd spend now and regret can be added to what you'd spend upgrading your set-up along the way. So a budget of $2000 today might be $3000 a six months/a year from now when you have more knowledge and developed some preferences. Don't be afraid to get into Eurorack. But temper your enthusiasm to avoid costly, early mistakes.
Edit: the Koma is nice... you can also add another semi-modular synth like a Behringer Neutron for like $300 and use its features as a Eurorack toolkit... extra ADSRs, extra filter, slew rate limiter, sample and hold, etc. Also, no need to buy a powered case. So for $300 along with the MiniBrute you've got enough kit to do a lot of amazing stuff.