Depending on how much work you want to do - gasket match the inlet and exhaust tracts; headers; larger bore exhaust; street cam; port the heads (head off for the last one). After that it gets REAL expensive. How much power do you want to spend?
Good - Fast - Cheap: Pick any two.
E23 735i; E32 735iL; E34 535i; 2 X E38 735iL (Oxford Green and Alpine White); R50 Cooper; R55 Cooper Clubman.
E38 Parts Car
E21 318i; E30 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i
I've just changed my wiper blades and now my reverse lights don't work. Should I change my fuel pump relay?