I can't recommend building a drum machine setup within a modular. The problem becomes apparent when you look at the problem purely from a fiscal standpoint.
For example, let's look at Uli's recent redux of the Roland TR-808. This gives you sync I/O, each voice has an output, got MIDI and USB, etc. Cost = $329.
Now, let's build a VERY SIMPLE module complement for doing as much as the RD-8 above (hint: it can't):
Now, that contains most of the RD-8's sound complement, clocking, the VCF and waveshaper, stereo mixer, and stereo out with a headphone preamp. Even trying to keep the cost as low as possible, this still comes in at $1182. That's a difference of $853.
How many drum machines does $853 buy, hmmm?