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.
-- plragde
Speaking money-wise and maybe also how fast I can get something going, it would be the better option at this point. Interesting to read that you had the same setup before moving on. So I am really considering it, but will first continue my research. I am quite hooked on the idea to start a modular system because I want to explore it and to create my own instrument. However, I realise that this is far from being done and I think that finding the sound you want can take years of experimenting, trial and error, and so on. I also saw the Bassline module and even though it is not the same as the db-01, it might be a good start in a setup.
@Lugia: Your comment is much appreciated. There is a lot of densely compressed information in there! I think at this point I have to re-evaluate my options and maybe start out with VCV-Rack, play around, see what I can get done in there and understand the process of building a modular synth in more depth. Then I might come back here with my second shot on a system. If I end up buying the db-01 I will post that here ;)