I'm new at it, but a decent gmaw will do just fine for guage sheet and tubing in mild steel. I had a 110 volt model and sold it to get a small 220 volt. I like my millermatic a lot more than the craftsman 110 volt it replaced. I tried aluminum once with my mig and wasn't overly thrilled with the results. You're talking about nearly trilple the cost for a gtaw with ac though. I didn't have that much at my disposal. It would also have needed more circuit amps to run. I sure you'll get some more qualified and detailed responses from others. One thing you could do is take a short class on basic welding. It'll go over the skills and more importantly, the safety issues associated with using whatever unit you get.

Motorhead,
I will second the advice and suggest formal training is never a bad thing, even if only to help your choice.
I will go a bit further, and as I often do, on a slightly different tac, and point out that once you produce a good weld there will be fixtures and other devices which you may prefer to fabricate for yourself to suit your own needs precisely. Don't under-estimate the scope of welding you will want to perform. Common advice, although by no means the ONLY advice, is to consider 250 amp GMAW the minimum (MIG) system. With it and a carefull selection of shielding gas you'll be able to weld nearly anything you're likely to encounter.

That said, it seems smaller equipment of around 125 A is in use all the time with good results in your kind of hobby. My suggestion would require a 220V 50A (single phase)service at minimum... this 125 would plug in to a 110V socket and be cheaper on that account alone. GMAW is a process which lends itself to production, though, and becomes a little cumbersome, at least in my own opinion, when called upon to be versitile. You might find your supply house loading you up with a lot of expensive accesories to get the job at hand properly completed... it may be more costly than you, at first, think... so do think it through completely and include filler wire, shielding gas mix, and other equipment such as special guns, liners, and such in the evaluation.

I believe GTAW (TIG) would present countless advantages for you, especially if you already have developed a proficiency and could identify a sound, used machine when you try it, and knew the features you needed. There are other threads. I think Bzzzzzzz was in the same boat about six or eight months ago and they're in the archives, but I'm not sure he's the one I have in mind... this individual wound up with a GTAW, and perhaps Bzzzzzz can help identify the welder.

Regards,
d