I agree http://www.mig-welding.co.uk is the site for welding advice.
I recently bought a Butters 181, and its more than capable on the thick stuff but welding anything less than 1.5mm you have to be really careful, it helps to use the technique described here http://www.mig-welding.co.uk/thin-metal.htm.
Guys on the welding site will talk about Portamig welders which will do what you are after - both thin and thick stuff.