Hey 4 Door . . .
I had the same problem with my E36 325i. When I changed motors, it sounded like it wanted to, but it just would'nt go. Come to find out, it was the ECU. Put a new one in and she started.
I also, checked my crank trigger sensor. Try that . . . .it's the cheaper of the two.
I hope this helps.
GOOD LUCK ! ! !
Skycity_BMW