All you need is a car running in good condition. No leaks, no loose stuffs and a helmet. however to compete in high degrees of racing you need.
Roll cage - ~$661 shipped from DGTrials (might as well since 2005 requires full cage)
Driver's Harness - $100 (you probably need a new one if you're more than 2 years out on SFI dates)
SA95 Helmet - $200
Racing Suit - $300 (single layer)
Fireproof Underwear - $90 (top and bottom)
Fireproof Socks - $15
Racing gloves - $40
Racing Seat - $400+, plus rails ($250?) (the rules say the seat must be attached to the roll bar/cage, but I find it difficult to imagine how to do this with a stock non-racing seat)
(DGtrials)