I wouldn't buy a specific marker to match your team colors. New teams have habits of starting then fading out, as well as significant roster changes. I would get the color you like and something that would go with anything.
Your budget does not have a lot of room to work with so you are going to be limited. Personally I hate the Ion, I don't like the way it feels and have seen many problems that make me question its reliability. As to your argument over back guy vs front guy guns, where do you base that from.
Number one in modern tournament paintball the sucessfull back guys are not just stationary turret guns, you also must move as well. Why would you want a heavy gun for that, just because you are a back guy. I play back but just last week I still had to take the snake (not my usual position to say the least, as I am not some wirey little troll

). A good player whether back mid or front needs to be capable of making moves on the field.
Number two, as a back guy you need a gun that is fast however now with all the 15 BPS rules, all guns are capable of the same speed. Reliability and feel are much more important. Now one thing that is more important for a back to look at is air efficency, since you will shoot ALOT more than your fronts. (That is why I loooove community paint at Tourneys).
Number 3 have you looked at most Pro teams, the front guys have the same markers as the back guys. Take the Russian Legion do you think their fronts or backs are hindered by having the Timmy, or the same with Dynasty and their Shockers.
I am a back guy and the two guns I use the most are a 2003 Shocker with a 2k2 Timmy Tadao board with Timmy break-beam eyes as well as a heavily upped Alias. Neither of them would be considered "too light" to be a back players gun. I have had DMs, e-bladed cockers, and other heavier guns and I can honestly say I never once thought "gee I wish I had a heavier gun". As a back I still want a light, accurate gun that allows me to do my job to the best of my ability without being a distraction to that goak.