i have a clarke 90 and its ok for up to ~2mm steel. i picked mine up from ebay for 70 quid.
regarding the amps side of things, you ideally want a welder that goes down to 25 - 30 amps for body panels, anything greater and you may start blowing holes in the piece especially if you are new to welding like me.
the mig welding forum is very informative and has a good knowledge base. there is even a tutorial section.
here.. http://www.mig-welding.co.uk/forum/index.php