I've got a Lincoln 100 amp and as stated its a good little machine.
But, if you want a machine capable of welding something a little heavier, step up to a Miller 135 or 175. It'll weld up to .25 inch effortlessly all day, whereas with the Lincoln 100, you'll be pushing it.
Lincoln likely makes a machine in the same amp ranges as the Miller, so check them out.
My personal preference is the Miller. The 18 year old machine I own has been in bodyshop use for all that time and has never let me down. Its getting a little slow as far as wire speed (too slow now for proper aluminum welding), but it keeps on going. I don't think its ever kicked out the overload.
You can use co2 for bodywork, but you'll be doing a good bit of grinding. As Stretch said, 75/25 is your best bet. I've never liked flux core for light gauge bodywork. Sooo messy compared to gas shielding.