Alex
No question TaG kart is the best place to start. Even if you want to move to shifters, you are better to start in TaG than jump into shifters.
At age 19, you are considered to be senior. Junior is 12-15, senior is 15+, master is 35+ or driver over 200#.
As far as engine choice, there are two that are most common and have the best support from the importers. The Leopard engine made by IAME (the largest kart engine manufacturer in the world and imported by Russell Karting and Italian Motors, both very good and strong importers) and the Rotax, ( Made by Bombardier, the largest manufacturer of recreational engines in the world and imported by 3 companies, SSC Southwest, F1 Boston and Cycle Barn, the largest motorcycle dealer in the country). There are others with strong local followings, but are definetly second to these two engines in terms of support.
The Leopard is a very good package and every kart shop here in Colorado sell and service them. The Rotax is sold through authorized service centers, of which there are 3 in Colorado, CRE Motorsports in Colorado Springs, Critical Zone in Grand Junction and us, Shockwave Karting in the Denver metro area.
Basic advantages are the Leopard is cheaper, wide power band, competitive at most tracks, you can rebuild it yourself, carburetor is easy to tune, and it lasts a respectable amount of time between rebuilds. One weak point is the starter which fails because of the vibration. Some have trouble with the clutch. The Leopard can run in Tag or Stars.
Rotax is a bit more expensive, parts are expensive but it lasts longer between rebuilds. Work doesn’t need to be done by a service center unless you intend to run it in RMax challenge races. Then it must be sealed. It is competitive at most tracks in the area. The Rotax can run in Tag, Stars or RMax.
To find out what all the shouting is about, I strongly recommend that you come to the RMax Grand Nationals starting in two weeks at The Track. Over 300 racers from all over the USA, Mexico and Latin America will be there competing.
By the way, we race both Leopard and Rotax depending on the track we are at.