I agree with drj81, get down to No Limits, get some time racing around with qualified instructors to help you out. There are heaps of Motorsport events happening in WA as pointed out about.
IPRA is great is you have a relatively unmodified car, but for people like me who've already gone too far with mods then WA runs a Street Car class which is pretty open.
The only really cheap form of motorsport other than Go Karts is probably to go HQ racing, but the thought of driving something with drum brakes gives me chills