Bit of a tough one to answer, it's quite subjective. Some people want handling, some want power, some want stance, etc.
A stage 1 map will always be a big improvement for little money.
R32 brakes are affordable and do a good job.
RNS510 or similar for tunes, nav etc.
Anti-lift kits help get the power onto the road.
H&R springs give a decent drop and ride nicely - coilovers offer adjustable height, and are great on a track.
Painting the skirts and black plastics on the bumpers tidies the look up.
An aftermarket downpipe with decat/sports cat helps the turbo spool quicker, frees up a bit of power, and sounds good.
K04 turbo and appropriate injectors is a tried and tested means of getting past 300bhp.
The list goes on!!!
