I have replaced my transmission with a rebuilt from a california rebuilder who specialized in bmw trannies. it cost $3500 (installed by a local tranny shop) and came with a 2.5 year warrantee [which i can't seem to spell right]. I've been very happy so far. I would strongly recommend going this route rather than going with a used unit.
To answer your question, eric (months now after you asked it) i came to understand that these auto trannies only go around 160k before crapping out. Good luck.