Honestly I'm probably one of the few people who hasn't owned or rented a Tippmann (went with a Spyder) so I'm not 100% certain what would come in their rebuild kits but most likely a cupseal and a couple o'rings like I said. As far as replacement, when it stops working basically. With the amount of time you're planning on playing as long as you give it a couple drops of oil every 6 months if it's sitting your o-rings will stay fresh enough that you might never need it.
As for barrel length you need to realize the 98C is a long marker as it is and would be ridiculous with a 21" barrel when shouldered. Stick to 12 to 14" anything much longer and your ball is fighting friction and efficiency is going to suffer. You could look for a set that includes a barrel kit like the Freak, Evil Pipe or similar or pick it up after the fact. The current trend is that paint is getting smaller and you probably wouldn't ever need half the inserts in your kit.
The real advantage of a kit like the Freak is the interchangeable backs so if you ever buy a new marker with a different threading $25 or less should get you a back and you'll have a whole kit for that marker.
There are also barrels like the Apex or Flatline (haven't used either) that trade accuracy for range which is counter to what you're looking for.
Don't forget the role that paint plays in the equation. If you're shooting seconds you're not going to have the same pattern consistently that someone shooting a midgrade paint will have. It pays to spend the extra money for better paint if you want to make the most of your opportunities.