The one I had my eye on was this 5.5W version, with a very large engraving area and open bottom.
http://www.ebay.com/itm/50-65CM-Mini...16.m2516.l5255

But I do see the point about it having a hard time doing actual cutting. Seems you have to make several passes.
The only real option in my price range for a CO2 is the typical China K40 you see on ebay.
It gets "ok" reviews, Im just not a fan of its design, being much smaller and having an enclosed bottom.
But, something I also thought, I might have a big issue with smoke on the open designs, since it will be much harder to vent without an enclosure.

The 5.5W one seems much simpler, and easier to maintain to me over the CO2, but maybe Im wrong?