My advice would be to read up on the mods the one make series racers do. They arent allowed to many engine upgrades and have to use stock brake calipers. They do simple and cheap mods and will out drive/drift you all day long. Do what they do and then drive the car. Counts for E30`s to.
Tyres, full decent alignment and some driver tuition will see you enjoying the car far more than an extra 5-10bhp ever will. Then turbo when you can/need/want to, no point spending a few hundred quid on "upgrades" that will do fuck all, save that money towards your turbo kit and learn your car in the meantime