I know that you should buy one with a full service history. If neglected the engines need to be rebuilt and this can be quite expensive.
There were 3 main model types the FB FC and FD qirh rhw FDs being pretty rare in the UK. The turbo engine is the best one in terms of tuning performance gains on the FC with the later FD getting a twin turbo option giving around 280ps of power. The FD is still considered by many to be one of the best handling cars around.
First up I would say breathing mods, so exhaust and induction kit. Then you can get a remap done or better still go with a larger turbo for more peak power. Also get a better intercooler and this can be front mounted at an angle - like the setup ascaudio has done in his. Its worth reading his thread in the gallery.
Suspension mods help a lot - although the car is really good at handling in its standard form.