Basically, to be legal you need safety equipment: 6-point cage, race seat+harness, window net, fire extinguisher, kill switch, and your safety clothing.
Beyond that, you are allowed to gut the interior, make some mods to the suspension, intake, exhaust and very limited performance mods to the engine.
I've laid out my mods to my ITA car when I built it on my site:
www.racerjake.com
Before you buy anything, get a GCR, but you can ask generalities here.
2 tips: 1. It's usually cheaper to buy than to build. 2. If you are going to build, find people who have built that car already to see what they did. (learn from the mistakes of others!)