Those are just different motor classes, different levels of power. That is how 1/10 scale racing is split into classes, generally the other rules are the same.
Stock is the lowest power class (but with modern timing ESCs is plenty fast). 17.5 brushless. (or higher)
Super stock is an intermediate power class. 13.5 brushless. (or higher)
Mod = modified, you can run basically any motor you want/can handle (just has to be a standard “540” sized for 1/10 scale).

Going by your avatar picture, be aware that a 3.5 would be way too much for an off road buggy. Even the really good guys are usually in the 10.5 to 7.5 range for the most part.

Be sure to check with your local tracks before deciding, so you have a regular class to run in.

Any of the name brand motors are reasonably close in performance. Trinity, Tekin, Novak, Orion, Speed Passion, etc. Grab one and join the fun, start doing laps and work on that driving!

As mentioned the other numbers, 27 & 19, are for the older brushed motors. Brushless is the way to go if at all possible.

Note that only 2s lipo batteries are legal for 1/10 scale racing. (From your other thread about a 3s lipo.)

Good luck!