Hey Lars,
I believe the Buildbotics.com project is worth it to be further developed. And it is worth to be used because of what it offers today. This includes also the fact that it is available at a reasonable price (with respect to performance/price ratio). And being able to use the Onefinity Controller hardware would be even cheaper – but as said above, I didn’t try it yet. But there is the original upstream Buildbotics.com hardware – and the open hardware pcb so you could even make your own hardware of it (just as Onefinity did).
What I would see as the first thing to do is to integrate the bbserial.ko kernel module into the DKMS and the Debian/Raspbian package system and to free it of its hardwiring to an ancient kernel source, as I described here: