with manual, you need to find engine, rear subframe (525i bigger than 520i), and rear differential (optional).
better find one that comes with VANOS. Coz the wirings of non-VANOS and VANOS are different.
Only find a 520i 24valves if it is Manual. If it is auto then no point, juz get a 525i auto directly (Coz you also have to find the manual kit).
Actually a 525i 24V VANOS with auto 5-speed is adequate for average use already (can tapau many stock cars already)... go test drive one before you making decision.