[boxhead]

It will depend on the lap times you do.
If quick, then your roll bar/hoop must be a set thickness and material.
Otherwise it is just a helmet and seat belts (as far as I remember)
Other items are good to have, but not a requirement.

Helmet you can get as cheap as $100 or so. But then you will also get the usual "how much is your head worth when buying a helmet"
Car must also pass scrutineering (so must be safe)

SO can be as cheap, or as expensive as you want.
I also bought a fire resistant suit, gloves and driving shoes.

Forgot to add, you will also need fire extinguisher and battery kill switch.