Well I have been playing for about three years now and have had all kinds of markers and guns. I have had anywhere from low cost guns to tournament grade guns. all have worked just as well for my weekend warrior type of play.
first you need to decide what type of paintball you play and where. Do you play indoors or outdoors?
If you play indoors and in a tactical type of senario then you might want to think about the size of yuor gun overall. most of the time playing indoors the problem is room and C.Q.C. (close quarters combat). You need to be able to swing your gun freely and there isnt much time to aim its just point and shoot. plus you want the gun to be light in the indoors so my suggestion for starters is to put a good shooting barrel on it ( i have liked the dye barrels) anywhere from a 10 inch barrel to a 14" barrel. i would also put a rear cocking system on it due to the bolt lever having a slot in the side (what a mess inside the gun you will have if you get hit in the gun there). also i would put a remote system on it to make it lighter and shorter.
If you are playing outdoors then i would do it up like mine. I have a tippmann 98 custom with rear cocking, car stock, remote air, and the flatline barrel system on it. revo hopper and it runs off compressed air not CO2 (cold weather). beyond this i havent added anything else but let me tell you with that set up there isnt a person out there that i have played with whether they be rec players or tourny players that have been able to reach me with paint before they feel the sting!