IMO if the classes are that expensive (we are pretty lucky here with all day fees at most shows), but I would do the Sr Stallion Halter class to see how he does, I would do Model also but keep in mind it is not a HOF accredited class just all star points, I would skip color he is a pretty color but since in R that is pretty much all they are judging its not worth the money (once again just my opinion) I only show it when offered the all day fee, if you get him driving I would obviously do that, you will need to determine if he is western, country, or pleasure (descriptions of each in rulebook), obstacle and showmanship are both good too, hunter/jumper (since you say he isnt great at it yet I would pick one or the other to start with so you dont burn him out, hunter is slow, steady, and pretty, Jumper is speed and clean rounds), and my personal favorite is Liberty as it is a great class to just let me and my horses unwind at the end of the day!
Like I said though that with classes that high in price, depending on what your budget is, you should pick and choose whichever classes you think you will do best in and also have fun in. Make sure and let us know what classes you decide on and how you do. Good luck and remember to smile and have fun!