Bit late to the party here, but a few thoughts:
1. Adams Gas is another one like Hobbyweld if you want to try. I use them for my welding stuff. Same sort of deal, no rental but a deposit. A "half height" bottle lasts me a couple of years!
2. For car bodywork, DEFINITELY use 0.6mm wire. The energy required to melt it is significantly lower than 0.8mm wire (just over half the cross sectional area) and much closer to what you need to blow a hole in rotten car bodywork! Most MIGs come with a double-grooved pulley for the wire feed - one will be for 0.8 wire, the other for 0.6. Just undo the retaining screw and turn the pulley round. You'll also need 0.6mm copper tips. The torch and liner stay the same for both.
3. A lot of people report better results with pure CO2 rather than Argon or Argon / CO mixtures when it comes to very thin stuff like car bodywork. I don't know this for certain, but I'm told the CO2 allows the heat to escape faster than Argon does, resulting in less heat distortion and less chance of blowing holes in thin stuff. I tend to use Argon for everything though, because I often have the opposite problem (welding 5 and 6mm stuff with my poor little single phase MIG, trying to get adequate penetration)!