What would be the best type of barrel for my Spyder Sonix paintball gun?

I have a Spyder Sonix paintball gun and was wondering what type of barrel would be best for it. First of all, do all barrels fit all guns?(I mean the part you screw it into.) I dont know much about paintball because I just got into it. My friends keep telling me the longer the barrel the better, but I am not sure. What does a longer barrel do for you and how long should it be for my gun?

What would you value this paintball gun and gear set at?

Thanks in advance for reading. I am wanting to sell this paintball set and need to know what is a reasonable asking price:

Airgun Designs 68 Automag Classic – all stock, like new used less then 5 times.
Quickload battery operated hopper
Standard Co2 tank with optional coiled remote line
Scott mask and padded gloves
Belt with paintball refill canisters (3)

All books/parts and even an instruction video are there.
The hopper is a Viewloader.