Chiming in w/ killernoodle, Tippmann is a very reliable company known for making markers that are both sturdy and good performers (were when i was playing a few years back, anyway).
The correct barrel length is dependent, as mentioned above, on your play style/setting. But, also remember to watch the bore diameter. There are all sorts of sizes to paintballs between the various brands (they're all supposed to be .68 caliber if I remember correctly, but you wouldn't know it comparing them), and matching a small dia paint brand to a large bore barrel, or vise versa, will leave you a very unhappy camper.
These guys have a rather large database of players' comments on various products:
http://www.pbreview.com/
Have fun, and keep that mask down
