The engine in my BMW 325 is really bad and I am thinking of replacing it with a 318 low mileage engine in a DIY with a friend. My girlfriend prefers the 318 engine as she says it will cost less to fuel the car since she is the one that drives the car more often.

Please what parts aside the engines will require replacement.

Please help as I will be going to pay for the new 318 engine by this weekend.

Unless you also have a engine control module engine wiring harnesses new dashboard for 4 Cly engine and necessary hard to get items like different torque converter and probably transmission too. Do It yourself I think not. Replace the 6 and start looking for a complete car with the small motor.