Back when I used to be more active and played Paintball quite a bit, I was looking for a backup marker, in case my main one went down. I also didn't want to spend a arm and a leg on a second one either. I also wanted a marker that I could tinker with and was compatible with most components on the market.
I ended up purchasing a Nuline Bruizer Outlaw marker. I think I spent around $200 for it, which at the time was about the normal price for a low end marker. This was quite a few years ago....circa 2001.
The Outlaw is a all aluminum marker. It has standard features such as double trigger, its reg allows you to use both nitrogen/compressed air, has a raised shark rail, custom drop forward, and a ported barrel.
I changed a few items, including the barrel which isn't the best in the first place. The regulaor worked well also. I would change the bolt, and a few other things. Another thing I liked about Nuline is that they included a hard carrying case. Rarely do you see this today. But it was a nice free added option.
I had very few problems and it worked great as a backup. Unfortunatly Nuline no longer exits and was bought up by WORR. But if you can find one of these online on EBay or such and want to tinker and you can get a good price on it, then by all means do.