Going to put my foot in my mouth on this one.... sorta.
While I still think a dedicated engine-driven power supply (ie. Bobcat) AND a Mig unit is a bit overkill... Miller just released a unit today that is the best of both worlds at a reasonable price IMO ($~2200). The key feature is that is is engine-driven, MIG setup, but can be plugged in as well

So you get all the benefits of a genset, but have a regular MIG at the same time. Caveat, it is only a 180 amp machine (plenty of 99.9% of auto work IMO) and the generator is only a 5000W (again suitable for all my foreseeable needs). Another downside is the 10lb roll capacity, not a biggie IMO, I use the 10 in my 110V machine, I don't use it much but I expect a roll to last the year or more.
A local welding outfit had one on display at the offroad expo this weekend here in town, it seemed to be getting quite the buzz from folks. Big enough to use in the garage, small enough to toss on your trailer when heading to an offroad event (competitive rock-crawling comes to mind).
http://www.millerwelds.com/products/enginedriven/renegade_180/