Yeah, you need to check your local track rules but will explain how our club operates.
Stock is loosely referred as 17.5T motor in 2WD Buggy
Super Stock is a 13.5T motor in 4WD Buggy
Modified is no motor restriction but all classes are limited to 2S LiPo
Stock motors can get very expensive if you want to be competitive, but for someone just getting started, having too much speed can be detrimental... I would just get a cheap HobbyWing combo and get LOTS of practice time.
Be sure to buy the most popular chassis that is raced at your club and ask for a copy of a setup sheet from a local pro, ask for help in getting your car properly setup so it's easy to drive.
I don't want to drown you with too much information but here's a good start to get familiar with setups:
Which club will you be racing at, can you share a link to their website?