I went the shifter route (ICC, then CR125) and my honest advice is go Tag or Rotax. Go get one and use it for a year. Buy it used, sell it used. You will have way more fun and won’t spend all your time fixing it, upgrading it, shifting it and going off the track!
I like shifters, but if I could have learned on a Tag I believe I would have had a much better time, driven more, and spent way less money.
Then I would have bought the shifter later (1-2 years later) and would have the experience to tune it and drive it.