.21's are the enginge of choice. think of this as v4, v6, v8. the 21's are v'6's I know you may think well why not get .28 well the truth is. The 21's are race legal anywhere and will whip any 28 you can buy. 28's are to torquy even for some truggys. They don't produce near the top end of a .21
A good beginner motor would be a RB S5 thats a 5 port. It's a good all around engine thats easy to tune and long lasting. It will perform well in a buggy or a truggy. They are about $229 run the 2045 pipe with it you want be dissapointed. i also would recommend GO Techs they are good engines. You get alot of bang for the buck. A GO 7port is only $209 and its a very fast engine. Some will say they are a little more difficult to tune. I would tell you that it's not the engine it's the person trying to tune the engine. I said those engines because they are both relatively cheap and cheap is good when your a rookie. A rookie should not buy a $300 engine to learn heck. You don't need a $3-400 engine anyway's to be highly competive even if your a vet. Spend your money wisely buy quality tires which is the 1st point of setup and racing.