First step is always consult your rule book. You may find that changing tire types (ie "streets" to "r comps" may change the class you run in. Same could be true with seats.
Sparcos are great seats and you can sometimes find them used for even more money savings. Spending the money that GT-3 seats command isn't worth it to me if the "savings" can be spent on tires, brakes, entrance fees etc.
You don't mention your experience, but the advice posted above was good. Check out others in your class and see what they run. Tires make a huge difference in how the car performs. Having a set of dedicated AX tires is not unreasonable
