Avoid sip welders like you would the plague. If you want to know anything about welding pop along over to this site here http://www.mig-welding.co.uk/forum/index.php
you will find a wealth of info over there and if theres owt else you need to know just ask
I myself use a murex tradesmig 191.
It goes upto 190 amps for the thick stuff but then down to 30 amps for the thinner,car body type of stuff.
The thing you need to watch out for when your buying a welder for car bodywork is to make sure the the machine will go down to at least 30 amps.
Some of the chinese cheapo machines on fleabay look good in the adverts but when you check the amperage they only go down to 60amps.
So basically not worth a toss for thin metal.