I don’t think you should see the Doepfer BBD as a normal BBD delay. If I’m right there is no filter for the BBD clock noise, which is an awfully harsh sound that lowers in frequency as you lengthen the delay time. So if you’re using it as a normal delay, make sure to add an extra lowpass filter just for the feedback path of the delay. It’s an amazing creative module for time-based fx and karplus strong synthesis, though - and if you decide to get the extra filter, it’ll be an extremely flexible delay!
Apart from that, the usual stuff people forget: attenuators, mixers, VCAs. There are quite a few sound sources in this rack, you’ll need ways to control them. I have only 3 monophonic sources in my current rack, and about 10 VCAs, 4 mixers and 3 attenuators. But if you start with purchasing the voice that interests you the most, you’ll find out on the way! Just don’t buy too quick and take the time to get to know your modules :)