You don't need a gun to get kills at XT. You just need to be really sneaky. I know I've managed to get kills when all of my guns have failed on me just with my bare hands. You can also ask the staff, the vast majority of my kills in the arena come from a 160 FPS springer pistol. I think you'll be just fine with that gun.
The upgrade packages that are mentioned on the site are just that. They are basically the usual M-15/MP5 package with some additions such as RDS' and tac lights. If you're referring to upgrading your own gun that would depend entirely on the gun itself. Not every gun can be upgraded reliably. XT sells upgrade parts that may make it possible to upgrade your gun and they used to offer upgrade service but I believe that they no longer offer that.
If you're serious about upgrading your gun you should first find out the brand and the exact model of the gun. Do your research and determine if it's really worth it. As TC said for indoors you probably won't need it.

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"Two Wongs do not make a Right"
XT Team - Task Force Wong; Tai-i Wong
-TFW Springer Weapon Assault Team (S.W.A.T.)