Believe me, this post is coming from knowledge and experience.
Mags are VERY tempermental on CO2. I wouldn't get one unless you're running nitro. I had troubles even with a remote and expansion chamber. The mag I use now is on nitro, and it works fine, albeit a few chopped balls..
That deal ain't a great one. I got my nearly brand-new Minimag tricked with new trigger frame, foregrip, diamond lab 8-hole mod, foamie bolt, Progressive barrel, Revolution hopper, 6+1, coiled remote, and Crossfire nitro system for $375 American. Of course, that was the deal of the century.
My advice to you is to get a new Autococker. They're only $325 american at www.paintballgeeks.com
and they're a lot easier to take care of than they used to be. The 2001 cockers are now just as reliable as Mags, and that's saying a lot. I've been a hardcore Mag user for most of my life, but I can tell that the Cockers are better now. Also, Cockers run pretty well on CO2, so you could keep your tank and use it if you don't have the money for nitro.