keep your car as a beater and buy a cheap 91' 318i/is.
im buying a 91' for 300$ sometime this month. all it needs is vaccum lines.
since your new to bmw's, a swap wouldn't be the best thing untill you have more knowledge about the cars....but thats just my opinion.
but if your just dead set on swapping a m42 in, it would be straight forward. since the bottom end is almost the same, but you might run into some issue's, but thats any engine swap. it could be done in a weekend if you were set on it.
expect to pay around 5-600 at the absolute cheapest, (im comparing prices from what i can find shit for)
as for the tranny, its a getrag 240, its a solid tranny, ive seen 24v swap's with the getrag 240. some have issue's with 2nd and 4th, but these cars are getting old, what do you expect?
as for your bumpers, take a power drill, and drill out the 2 shocks on each one, then kick them in. its called tucking them, it makes them look A LOT better. if they dont budge once you've drilled the shocks out and kciked them, just slowly back/drive into a tree or pole centered up on the bumper.
just don't hurt your self in the process

cheers,
will