I haven't had any firsthand experience with ZF autos, but my uneducated guess is that you've suffered failure of the transmission from revving it in neutral or park - that will, with absolute certainty, kill a ZF if done for more than a few moments. I'd recommend an AW conversion, you'll need the driveshaft and OD relay circuit (or just modify the OD solenoid to always allow OD). I have no idea if the crossmember's the same or not. I think it's the way to go, even if you get a good BMW ZF you'll still need to swap bellhousings. The AW is easier to find good, used, and cheap.
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf, 90 740 Rex/Regina