I had a DB-01 and Digitakt before I started buying modules. Honestly, I think you should get the DB-01 and work with it, then see what you are still missing and want from modular. It will be expensive to build up modules that come close to what the DB-01 offers, and the DB-01 has three CV inputs (in addition to clock in/out and MIDI in/out) and a hot output, so it can be easily hooked up to a rack in future. Erica Synths does have a Bassline module (I suspect it shares much circuitry with the DB-01) but it lacks noise, synced LFO, and most importantly the seriously playable sequencer that really puts it into 303 territory. I know this is a modular forum and I should be talking up Eurorack, but sometimes the alternative is better.