S50 is the motor from an M3 between 1995 and 1999. If you are looking for cheap, a swapping a motor isnt going to be the best idea, unless you buy someones already complete project. If you are on like super budget, get a clean older e30 4 sedan and swap in an M30. The power those put down is BRUTAL, and they respond well to boost (350+ hp on around 7lbs)