Buy a used etha or an axe, i pick the etha over the axe mainly because it is eclipse gun and has only been out for a short period of time. Their operation and design is very close to one and another as well. You can pick up a used etha or axe for under $400 but be weary of the axes as i see a lot of abused ones for dirt cheap. Then use the rest to pick up some new spectra frames and a lens for your proteus, a loader and perhaps a new tank as well. (These guns wont use co2 as i would assume you already know). For new loaders you cannot go wrong with any halo 2 that you can find.