If you want to road race, you should get a set of coil-overs. This will give you the best street/track suspensions. HKS is about the best out there for the Supra. Tein HAs are good as well. Prices are 1300-2800 depending on what you get.
Best spring rates I've found to be good for both street and road racing is 650 lb/in front and 375 lb/in in the rear.
Take care
Mani