If you spent about $5-10k you could get into a fun and safe club car, mx5, s13/14/15 Skyline or even a Galant lol.
You need good brake pads, brake fluid, semi slicks, half cage, seat, harness, fire extinguisher.
Sure you don't need all that stuff, but if you want to have fun and not break stuff and be safe, its pretty much a gotta have list.
You gotta decide how serious you want to get, best to decide on the outset as it will cost you much less buying something ready to go.
Likely smart move is buying someone else's track car vs building your own.