Stock 2wd buggy is 17.5
Stock 4wd buggy and 2wd stadium have been 13.5 lately
Stock 2wd short course is 17.5 but many tracks are going 13.5
4wd short course uses 1/8 scale motors, Kv rated. I believe it's generally an open class
all stock classes are blinky. Stock racers tend to go for light weight and low rolling resistance, as well as getting battery internal resistance as low as possible.
Mod is any turn motor, and ESC timing is allowed. Weight, rolling resistance and batteries tend to matter less than in stock, but accelerated wear on everything is a draw back.
I race stock because I can't drive well enough to handle mod, and stock tends to be more popular.