Before you decide on models and brands, make sure you have local parts/tech support for whatever you buy, the best welder in the world, does you no good if you can't get parts or service locally for it. I have bobcat myself I bought used with only about 150 hours on it. First thing I'd do is set a budget you want to spend and go from there, if not you'll fall in love with the 10k diesel somewhere used and then not be able to buy it anyhow, remember each model is only a little more than the last, before long your into twice the price range you can afford or even more. Every model and price range has its abilities and drawbacks its up to you to figure out what "you" need and drawbacks you can live with.
As for air gouging, Willie is right, I air gouge with my bobcat and the limiting factor has always been the air supply, not the welder, yes there are welders that do it better and faster, but most will do it.
As for used machines, I've been down that road and most are far overpriced with many problems to contend with compared to new machines, when you compare used to new and hour life expectancy, unless you can find that one good deal, new is almost always the better way to go, then you know what you have and how its been taken care of and its history into the future.