TaG karts typically weigh 175-190 lbs, so with race weights for TaG Senior between 350 and 375 lbs, and racing gear around 15 lbs, that calls for a driver weight between 145 and 185 lbs. If you get a Rotax, Rotax International race weight is 365 lbs. Talk to the race director and see if they’ll let you race as an over-age Junior – you’re a lot closer to the Junior weights (320-340). If not, take a look at the race weights and choose an engine that has one of the lighter values. The good news is that the less powerful engines (EasyKart, PRD, Leopard) are usually cheaper. If you’re brutal on equipment the Rotax Evo warranty might be worth it.
I’d suggest looking at the results of the last season at the tracks you intend to race at to help choose a chassis brand. Most of them are good these days, including CRGs, Tonys, Birels, Intrepids from Italy, Arrows from Australia, and Margays from St. Louis, Missouri.
If you intend to run a shifter later, you’ll want a chassis available with front brakes. You’ll also want to look around for a spindle/hub/front brake combo for your kart in the eKn classifieds.
Renting a Briggs 206 or Clone kart for your first kart weekend before you buy your own will allow you to get used to the race weekend and get through your first race starts in traffic without having to worry about a kart that’s doing its best impersonation of a cruise missile.