Ive built up a track car which was expensive. Ive also bought a built up civic with a K in it already and simply bolted in a cage. I am having way more fun in the bought civic over the built one, because it was low cost mainly.
Safety is needed no matter what car or kart you drive, pay for safety, be it helmet fire suit or what ever else will keep you alive or a few more seconds to exit car quickly.
Lastly, have a great time. Talk to others and learn what you can. Help others that need it at track, it will come back x10 more when you need it.