A standard 3 point seat belt is sufficient.
All competitors differ on what they drive, and how they get that vehicle to the rally-x. I drive up my daily drive, and race it at the events. Other racers have purpose built cars that they tow back and forth. Its all really what fits in your budget. If you're racing your daily driver, you just have to keep it in the back of your mind on race day, that you're going to need to drive that car home, so not to push it to the full limit. This doesn't mean that you can't be competitive. With my daily driver, I've been Overall Class 4 Champion twice, and just came 2nd overall in class 4 this year.
Insurance...Well if you break something at the race, your insurance isn't going to pay for it

I don't tell my insurance and realize that if something breaks, I'm paying for it.
Rallycross is A LOT of fun. Its not for everyone, but I thoroughly enjoy it! You meet great people, its a really open sport and learn lots about vehicles from how to drive them to the limit, to how to fix em