autodark is awesome!
I love my autodark mask, it was some off-brand but still decent quality unit. I can't quote the # shade it is, but I generally use 2 super bright halogen lights so I can see clearly the area were I am about to weld. Otherwise, they are too dark to see. I don't think I would ever go back to conventional for auto body work, as it is not continuous welding but alot of stopping/starting, and flipping a regular mask gets annoying very shortly.
Start with thicker stock then work your way to thinner. MartinSR has everything covered.
MIG is definitely the way to do autobody. TIG works, and skilled guys can gets utterly awesome results, but it is SLOW.