i picked up an engine for $800 and the cost for the install ended up at around $3000.
w/a new flywheel, clutch, guibo, belts,hoses, the list is big, but....I HAVE NEVER HAD A PROBLEM SINCE.
bitcore, switching to a larger engine is never about being economical,....its about fun!!! 4-5k is alot for an engine.
if you felt like it, you could drop in a M5 engine, but its gonna cost you!
CDSystad where do you live? my mechanic is a custom porsche racecar builder, and his work is amazing on BMW's. if you need help finding an engine for a little less $$$$, let me know.