All consumable parts would be in summary a good start, of which fluids and belts as you have already mentioned. Further to that, you could check hoses, ignition coils and spark plugs and perform a compression/ leak-down test while apart.
Is it heavily modified or stock? Personally I wouldn't bother going way over the top replacing a whole host of parts, since you can begin replacing parts with upgraded or new parts as you go e.g fuel and ignition, alloy radiator, turbo etc.
Provided that the engine is in good shape (as tested from the compression/leak down) you'll know your working from a good base. Maintain it (oil/ fluids) and upgrade supporting mods (injectors, driveline bushes/clutch) as required to meet your goals (power/ application: street, circuit, and drift)