If you are going to buy a MIG--Dont skimp and get the cheapest thing out there.
Trouble with the cheapos is the wire-feed system often isnt up to the job, will jam, cause uneven wire feed (And horrible welds) as well as the quality of the torch and liner will give you issues.

For car-body welding, you'll need a MIG with a current range from 25A to say, 140A, and able to use 0.6 and 0.8mm wires. Its best to use an Argon based MIG welding-gas rather than the 'gassless' wires, as this gives a nicer (and for a newcomer) easier weld.
My MIG is a 'Cebora' Autostar 190, perhaps a little overkill for autobody work, but as its pretty robust is an easy machine to set and use.

Here-- MIG Welding Forum

is a MIG welding (and other types of welding) Forum that has tutorials etc to help a newcomer to welding and how to. There's loads of advice/help on there...
--Like everything, Practice makes perfect but the best tip I can give--Make sure the steel surfaces where the weld is to actually be laid down are as Spotless of rust and paint as possible--A few minutes with a grinder to polish up make a Huge difference to the finished welds....

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Alastair AKA H.C.II South Wales, U.K. based member

W123, 1985 300TD Wagon, 256K,
-Most recent M.B. purchase, Cost-a-plenty, Gulps BioDiesel extravagantly, and I love it like an old dog.

W114, 1975 280E Custard Yellow,
-Great above decks needs chassis welding--Really will do it this year....