Agree with TJ (as usual). My background was similar to yours. If I were you I wouldn’t worry about moving to shifter any time soon. My skippy instructor said the next step up from a 125 shifter is an Indy car, and he was not exaggerating much. Will Power–yes that Will Power–showed up at the Supernationals kart race in Vegas last month racing TaG Masters (the class you’d be in) and finished 3rd. Didn’t win a single heat. So I wouldn’t worry too much about TaG being too slow or too easy and needing to get out of it and move to shifter after a year.
Engine selection likely will depend on what is being raced at the track/series you chose to participate at/in. Both X30 and Rotax are the present “flavor of the month,” and they are roughly similar in terms of performance. Note: Rotax just introduced a series of upgrades, so make sure you get the upgraded engine and somebody doesn’t sell you an old one without full disclosure.