Personally, the only thing that matters is your paint to barrel match. When I first started, I was told that running a certain length barrel had it's advantages. Most people told me 14" was the way to go... I have since found out, my 12" barrels are just as good, if I have the proper paint to barrel match.
If I were you, I would save up and buy a barrel kit, such as; Evil pipe bomb, Smart Parts All American, or Smart Parts Freak or Freak Jr. Reason being, that this would have most of your barrel to paint matches set. Basically, paint should fit slightly snug in the barrel, and not fall all the way through. Most fields even require you to use their paint, in order to play.
Each field uses their own paint, thus varying sizes of paint are used. Get the proper tools to fight in a war, otherwise just go home!
And the best tool you can have, is knowledge!