I just realized I missed a couple of your questions in your original post.
Parts – if you get an ICC, the parts will have to come through a dealer for whatever particular engine you have. ICC parts are pricey. If you buy a stock Honda, you can get parts from any Honda motorcycle dealer in the country, and there are hundreds of them. Honda parts are cheap. If you get a Rotax (DD2 or TaG), you’ll have to buy parts from a Rotax dealer. Rotax parts are pricey, but you generally don’t have to replace them often.
Starting a shifter – there are no starter mechanisms available or legal for the 6-speed karts. You have to bump start them. Usually this means having somebody push you. You can do it by yourself (take off at a run pushing the kart, then jump into the seat and start it) but that’s not easy to do (worth practicing in case you need to do it during a race though). The best method to push start a kart if you don’t have a buddy with you is to ask somebody else in the pits nicely 😉
The DD2 is easier because it has an electric starter on board just like a TaG kart.