Hey Newbie,
I have always enjoyed motor racing (watched it never participated) and last year was my first year in the sport. I got into it after going to IMI for a bachelor party and (also) Sam Walls worked out at my gym. I agree with the things that are being said and i’d like to add a few things to the mix.
I bought a used GP/Rotax package and was glad i bought used. Would I have bought new? I don’t think so because I didn’t know enough to make a good decision, Then to be stuck with something i wouldn’t be happy with later. But that is me and I am cheap at times 😆 knowing what i know now I’ll buy new for my next kart. Do you want to race? Are you doing this for recreation? Those are questions you have to answer.
Shifter versus the Touch and Go (TAG) Classes? How deep are your pockets? Remember this: Have you ever seen a pilot go from a Cessna to a 747? Did Juan Pablo Montoya go from karting to Formula one? It is the same with a shifter versus TAG. Shifter is the top of the ladder.
As for kart chassis and engine packages get something that you like but more importantly get something that has local support. No good getting a Tony Kart/ Italsystem when there are no dealers here. Same for the engines. Personally i like the zit engine (Rotax) but again that is personal preference. I think that all engines have thier plusses and minuses. Your challenge is a)how much is the rebuild when that comes b)What is my engine’s weakness (no one engine is perfect) some are great low to mid range others great mid to high range in RPM’s.
Between the kart and accessories it cost me about 7,000 to race last year. (Dennis cost more because when he wrecked they were doosies 😀 ) (And I didn’t go to any out of area races other than Steamboat)
Go to any track here in this state and ASK QUESTIONS. Everyone here loves to educate anyone with a pulse who is curious! They’ll tell you why thier chassis and engine is better than XYZ’s piece of junk. Seriously if you don’t ask then that is your fault because we all are happy to give you knowledge and the more that race the better the competition.
Good Luck and hope to see you at the tracks! 
Mike Jansen
rotax #27d