I'm in the processing of building a small drum box myself, which you can find here: 
Take note - it's a very small case. I would not advise that at all to anybody starting with modular. My advise is usually to start small with the modules, but big with the case. Mantis or 7U, as mentioned above.
The Constellation is also a Euclidean sequencer, but next level. You can layer Euclidean patterns in many different ways that I don't understand yet myself. So the purpose of this very small rack is to focus myself on getting to know that sequencer.
I'm quite a fan of the Rample. 4 channels of samples. Mostly drums, some complete drum breaks, some silly or crazy, some very useful but not drum at all (e.g. piano or a singer). Ro'ved, which is a Plaits clone, is also interesting for drums, particularly when combined with Traffic. I'm definitely getting that module at a later date. Note the FX Aid also and the filter. Those are a bare minimum for any rack. You really want some sort of reverb to add presence, or a delay. FX Aid has plenty of those. There may be better ones out there, but FX Aid is a very good starter module. You can make your own selection of effects and play with them for a while, then select new effects to get to know those.
I've also added a sample-and-hold. This AfterLater one, or the Intellijel Noise Tools, are very handy in 1U. Of course you can find other modules in 3U as well. With S&H, you can add some variance to your sound so they sound less robotic.