[Sakudog]

Track Time
Coaching
Tires
Brakes
Fluids
Track Insurance
Safety (Cage, Harnesses, Great Helmet, HANs, Seats)

IF you still have budget left after that (you're doing something right) then you might want to consider some AP Racing brakes as your upgrade. The car has plenty of GO, it does however, run out of STOP if pushed really really hard.