what size to use

I have a Lincoln 140, and it works great. I used it for my notch, but I would recommend useing solid wire and gas its alot cleaner than flux core. I believe the Hobart 140 is the same machine. Its more than enough for sheet metal, and light frame work.

My suggestion would be buy the most machine you can afford,well to a certain point anyway,You want a MIG and I would suggest 220 power as opposed to a 110,the smaller ones will work but more power is always better,somewhere around 200 or 225 amp should do anything you would want to do on a car and then some,you can always turn down but you can only get as hot as the machine will allow! Miller and Lincoln both make good equipment and have good resale due to the name but also it's easier to get accessories,consumable and repair parts as well,I would also suggest solid wire and gas shielding as opposed to flux core as it's more versitle,of course flux core has it's
uses as well such as out doors on a windy day etc. I wouldn't be opposed to used eqpt either but would shy away from an industrial user
like a welding shop or someone like that as there's a good chance it could be worn out,home owner's,hobby shop,people like us,just so long as it looks like it's been taken car of and of course being able to have a demo doesn't hurt either,bottom line just like anything else,
some studying and shopping around and not being in a real big hurry can save you some serious coin,well hope that helps,Have Fun! KillerDave