If its purely car body, then anything from 20-50 amps, but of course there could be chassis work which means up to about 120 amps. are you ever likely to want to weld thicker than 3mm chassis section? if not something like one of the clarke machines would be ok, this one for example www.machinemart.co.uk/shop/product/details/135te-turbo-mig-welder/path/diy-mig-welders
goes up to 130amps, plenty good enough for up to 4mm, goes down to 30amps which is fine for car body panels.
