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 (X 2 - 1 Alpine White and 1 Glacier Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E30 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i