Lots of ways to go. Again, stop at a track and ask questions.
Used will cost $3-4k for the kart but it might include some extra goodies like a stand or exxtra wheels etc.
You can get good karts from any of the dealers around here, but the cheapest “new” package is probably the Adiamo from Tim at SAC for around $4,500. You can get these and other great karts for about $3,000 used, or you can go for better packages from SAC, Billet, Shockwave, IMI…
Get out and get some laps, then try some races. You will have a lot of laps between the time you get hooked and the time that you will be able to know what subtle tuning features you want in a kart. And regardless what you will want, more lap time will be what you need.