Currently doing exactly this. If you're gonna do it right it's going to cost you. Alot of it will depend on your own viewpoints and level of risk taking.
You can use the car as it is on track. All you need is a helmet. But you might decide you want buckets to hold you better....buckets lead to harnesses, harnesses lead to cages, cages lead to padding etc....you don't HAVE to have a fire extinguisher nor do you have to have fireproofs, gloves, shoes, HANs devices etc. But you may be the sort of person that wants to do something full on.
I've gone pretty full on with mine but left some interior trim so that I can use it on the road and still feel like I'm in a car...also left it taxed so I can get to the track without a trailer. But this will mean some self restraint when driving it so make sure I can get home!
Also think about where you would track it. I've got Donington Park about 10mins from my house, this makes the day shorted, the fuel bill lower and the hassle less should it break..