Not Move in a gear

Hi, my friend and I just picked up a 840ci. Seller said there is automatic gearbox problem. What was wrong:
You could start the car, put car in any gear you want, but car would not move. The revs was maximum of 2200 does not go any more. When car on "park" and you accelerate, car tries to move but it feels like you hold brakes and trying to accelerate while car on "drive", but that is actually on "park"
Next:
We bought gearbox the same as was in there 5hp30.
gearbox done only 86,000 miles.
We replaced old to the one we bought and car was running good for about twenty miles. Then I felt like it was hard for car to drive, and would not rev over 2200.
I drived it home, stoped next to home, tried to reverse to park it and car not moved no more.
what we think happened:
the oil pipes from gearbox to car are blocked and oil not circulating and not cooling, gearbox overheated and gone.
Any advice?
What we need to check?
Or anything we need to do?
Any help appreciated.
Thanks a lot.