ex – your questions have been hashed and rehashed on this forum in the recent past. If you go back and look at some of the other newbie questions your ?’s should be answered there.
Bottom line – Tag is the way to go for several reasons 1) Cost of entry 2) Learning to drive a kart 3) Learn to setup the kart without the motor being one of the biggest factor and 4) Tag class is going to be the largest fields at the track on any given weekend. The shifters have the best Top 3 drivers in the state; Ryan Bailey, AJ Noud and Sam Walls but the Tags have the better racing from front to back.
Motor package that you choose for Tag really depends quite a bit on how much you personally weigh. The actual kart without driver weighs roughly 160 #’s. The CSC weights run from 375 to 395 based on Tag usa +20. If you are a heavier driver, say 210 +, then it would make more sense to run Biland, Sonik or Itali. If you are a lighter driver <170 then rotax would be the way to go so you won't have to bolt on more than 40#s of weight to your kart.
Good luck, you will have a blast no matter which direction you go. Kart racing is an awesome soort and the racers in Colorado are some of the nicest people you will ever meet.