First off, I think you'll be better-served by going with a Tiptop Mantis cab instead of the Arturia. They also expand easily; Tiptop carries an extension bracket for those that lets you add a second cab above the first one. And power-wise, the Mantis uses a beefed-up variation on Tiptop's uZeus which is built into the cab; the Rackbrute requires 5 hp right off for its P/S. But the best point: 104 hp x2 for $335, as opposed to the Rackbrute's 88 (or 89, depending on who you ask) x2 - 5 hp for the power for $359.
The other point here is that trying to build a "drum machine" into a modular rig is a losing proposition. It requires that quite a bit of the space go to voicing and sequencing modules, which then diminishes the space available for synth modules...so, in the end, you wind up either with a huge and spendy system, or a more sensibly-sized result in which both the synth and drums wind up being somewhat compromised. And then...the cost!
OK...we'll take the obvious drum machine function modules, namely the Euclidean Circles and the QD, and check those prices...which come out to $772-ish, depending on the exchange rate. Doesn't sound too spendy? Well, to my immediate left as I type this, I've got one of Uli's 808 clones, which nails that sound, adds a few useful functions that the original didn't have, and puts all that in your hands for $329. And if you think that's not sufficient, add another machine and lock it up with the first one. In the end, using these purpose-built machines is the RIGHT move; you cannot replicate the RD-8 mkii's functionality in Eurorack until you're on up in the $1k+ zone. It might seem more convenient to put it in the modular, but in the end, you lose too much and pay too much for that to be tenable.