Marlon said:
I can see the attraction, I watched Mike (DLV) welding up my modified exhaust and it looked great fun
OP - The best bit of advice I can giver regarding mig equipment is, within your budget buy the best USED machine you can and NOT a "hobby mig" they are literally, ALL crap! Even the "cebora/snap on" hobby range, they all suffer with undersized wire feed motors, and poor control of same. It is VITAL that the machine can keep the wire feeding at a stable rate, without pausing, otherwise you are fighting a losing battle!
As Fred said, he went from a hobby, to my old Erfi Rekord, and when I say old, I think it was a 1989 model? BUT, the wire feed motor is the size of a tea mug, and all of the components in it are metal, not plastic. Her's the thread, quality machine!
http://www.z4-forum.com/forum/viewtopic.php?f=42&t=96489&hilit=mig
I only sold it because the Wife bought me a Snap On Pro-Mig 180, which is a bodyshop rated piece of kit, not cheap at almost a grand, but a fantastic piece of kit.
Ebay is your friend here, but some sellers are reluctant to ship a welder, in which case, ask them if they'll pack it for you to arrange collection.
AVOID anything by Clarke, the cebora/Snap On "pocket mig" and anything "gasless" here are some I just spotted, ALL of these will give you MUCH better results than anything you can buy new for £300-£400..
http://www.ebay.co.uk/itm/transmig-mig-welder-/152694728466?hash=item238d50a712:g:e5MAAOSwFdJZsUz-
http://www.ebay.co.uk/itm/Mig-welder-murex-tradesmig-191-/112553490217?hash=item1a34b5f729:g:fQYAAOSwQPdZrnRW
http://www.ebay.co.uk/itm/butter-aw201-mig-welder-/322723141851?hash=item4b23cc58db:g:5goAAOSwTxpZsZk7
Personally, I'd go for this one;
http://www.ebay.co.uk/itm/Murex-Autolynx-161-MIG-Welder-/172855491790?hash=item283efd7cce:g:xK4AAOSwb3NZsSY1
Proper mig, that's had light home use, just what you want!
Mike