I am going to have to offer my non-professional opinion, born of experience here:
A 120vac MIG is more than up to the job for welding lock boxes to connex-container steel doors.
I built three jeeps I have beat the hell out of with my Lincoln 3200HD, welding from 1/16" high-tensile steel sheet (jeep Cherokee monocoque) to 1/4" structural sub frames and OEM chassis (S10 solid axle swap, bumpers and axle bracketry). I have had no problems at all using (flux core E71T-GS) and (ER70S-6 with CO2).
Metallurgically speaking, I am sure this is likely not optimum, but I have never had a problem with these builds and have winched many times without problems, several hard pulls.
Also, used a Lincoln AC225 for some parts of these builds, using 6013, but those welds are by far in the minority since I don't have ready access to 220vac.
Since the OP is new to this and just wants to get a task done and then play with/learn a skill, I would lean toward the stick machine and look for one used on craigslist. I paid $125 for my AC225. Recommend staying away from the Harbor Freight welders as they can be hit/miss.
Also, since presumably the OP doesn't have the ability to do these welds in flat position, I suspect the hardest part will be dealing with the out-of-position nature of this welding task. I think I would run them uphill to keep the slag out of the joint, and clean well with a grinder first. Probably not strictly necessary owing to the rod selected, but you can never clean a weldment too well before burning in.
My experience, and again I am not a professional welder but I have actually welded onto this material at work with the MIG mentioned above.
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JAFO screwed with this post 01-28-2013 at 06:06 PM