Honestly, I wouldn't put the Model D in the Rackbrute. First of all, it already has a cab and that cab takes care of its power needs. Secondly, it takes up a lot of room, and given the goal you're shooting for, the Model D gobbles up a lot of the rack space you'll need to make that work. Wanting to 'do it all' in 2 x 88 hp is a bit of a tall order...not wholly impossible, but if 70 hp is being used by what's basically a monosynth that can just as easily stay where it is, you probably won't be getting close to that notion.
Third: be extremely careful about which synths to sell. Consider that some of those, if you got them relatively cheaply, probably will not ever be at your disposal again. Case in point is sitting right behind me, and only cost $450 plus a couple of days driving when I got it, and while I could easily pull down a cool $18k for my Yamaha CS-80, I have been balking at it ever since the idea arose...because I will NEVER, EVER find one of these things at an affordable price again! Even if that revenue was 100% channeled into studio equipment, losing that particular synth would be a helluva hole in my equipment. In contrast, I was able to get another MS-20 (Mini), a few others I sold in recent times are redoable in more reliable forms (my PPG setup, for example...Waldorf's Blofeld Keyboard is just fine as a functional replacement, plus Wolfgang Palm's doing some wild stuff now with mobile devices), and one I regret selling can be regotten cheaply enough.
The 'Brute rig is one I've been working on as of late, although the eventual result will be a 2 and 2S with two 6Us (maybe), so this is pretty familiar turf. So, one caveat I would make is that it might be a good idea to get the Minibrute 2 first, pair it with the Model D, and then see what needs present themselves. As for me, I'm familiar with the Minibrute's architecture, so I know what needs to be paired with it to get it to go in the directions I want, but if you've not had a lot of experience with patch-overriding a synth's internals, I would really suggest doing that first before dropping a bunch of money on a Rackbrute 6U and a module compliment, then discovering that, wow, none of this does what I had in mind, so now what?
Also, having just discovered modular synthesizers, you might want to observe first. See what others are doing, research what the 'classics' had, then sort out the various strengths and weaknesses you're going to gradually notice. For example, why did Uli Behringer use (cough, cough, 'steal') the Minimoog's overall basics to come up with that Model D? On that one, if you know why the Minimoog was set up the way it was, you might see a couple of other reasons why I'm saying it doesn't go in the Rackbrute.
And remember: what you're contemplating here involves blowing several grand, ultimately...yes, even in that small a cab, if you want it set up correctly. Treat this more as building an instrument that you're likely to use for several years, not something you can toss like a Volca or some such.
Not trying to throw water on your enthusiasm, mind you...it's just that you're wandering into an area that's INFINITELY deep that makes a read through the Sweetwater catalog look like a cursory glance at a Hallmark card, and I'm just noting that you might want to slow a bit here and find out what all of this not-bought-yet gear can and should do before dropping a bushel of moolah on it. Make sense?