Stuff that must be bought new for karting:
1) Helmet
2) Rib Protector. I bought ones that cost between $80 and $150 for years and was always sore by the final. It turns out that the $21 Z Racing one (sold by East Lansing Kart Track) is the one that works for me.
3) Tires before qualifying in any race you want to matter.
Stuff that you should buy used:
1) Stand 2) Transponder 3) Kart Suit 4) Valhalla 360 or Leatt Brace (get one that wasn't used "in anger" of course!) 5) Kart. The right way to buy a used kart is to buy the 1st, 2nd, or 3rd place kart as soon as it gets past the scales. 6) Trailer. If you get an enclosed trailer that's the right size you don't have to unload when you get home if you've got a decent way to secure it. You'll come home at 0100 if you had a good day. 7) Shade Tent. 8) Copy of Memo Gidley's "Secrets of Speed"
Tools you need to buy:
1) A REALLY good set of T-handle metric Allen wrenches. Beta look expensive but you will use them so extensively that they will cost about half a cent per use. 4, 5, 6, 8, 10 mm are the most common sizes used.
2) A REALLY good set of metric Allen sockets. Snap-On, secondhand if new's too expensive.
3) Metric reversible gear wrenches. 8, 10, 12, 13, 14, 15, 17 are the most common sizes.
4) An Acceleration Karting axle removal tool of the appropriate size. Don't attempt to cut or hammer an axle out any other way. It'll mushroom and the bearings aren't cheap.
5) A chain breaker.
6) A Zyglo kit and grinder with a paint removal wheel. The #1 cause of bad handling in karts is fatigue failures. It's also the #1 cause of DNFs.
Engine-wise, I can't recommend any of them without fearing a pitchfork through my back after your latest blowup. Just accept that the engineering was done on the cheap 15 to 30 years ago, that the volumes are too small to merit redesigns for modern manufacturing, and no matter what it's still a LOT cheaper than running any modern high-performance four-stroke engine at up to 3 lateral g acceleration. "When in Rome do as the Romans do". If everyone runs Rotax at your track, get a Rotax. If everyone runs the Yamaha KT100, get a KT100. A Briggs LO206 will go fast enough to make the chassis and driver matter. Parillas are nice and you get to deal with Jim Russell or Lynn Haddock.
You can go wrong buying any chassis if it is bent, cracked, or worn down on the rails and crossmember. CRG, OTK, Arrow, Haase, and Parolin build good karts and you can get the parts from dealers or importers. I was in the Margay factory in St. Louis, Missouri a few days ago and they build kart frames so well they don't have to be individually realigned after assembly. The price and quality of Margay machinery is up to the standard of other modern American industrial products. If you can't find a good kart locally, karts are best shipped by Greyhound Bus.
You will find hundredths working on your engines, tenths working on your chassis, and seconds working on your driving - if nothing's busted.
Carroll Smith asserted that racing an open-wheeler was always cheaper than racing a sedan of equal performance. I'll stick my neck out and say that you can RACE a kart regionally for what you'd spend doing TRACKDAYS in a car.
You need to be in excellent shape. You'll be able to race a day as competitively as you're able to play whole basketball or ultimate frisbee tournament, a doubleheader tennis match, or run a half-marathon, or cycle a half-century, or swim three miles.
I'm returning to kart racing this year.