For £250-300 you could get a good second hand sealey 180-200amp MIG with a euro torch with a 3-4mtr lead instead of the 1.5-2mtr toy torches. The hobby type 130-150amp MIG's have a low duty cycle whichs means running them at full power will cause them to over heat and drop power down to 2/3 the amps, not good when you only have 130-150amp to start with. If you are using 2.5mm wall black hollow section all the joint ends will need to be weld prep with a 2mm 45' chamfer and a slow hand.
http://www.ebay.co.uk/itm/Mig-Welder...em19c742 6479
http://www.ebay.co.uk/itm/MIG-Welder...em2c5b48 b7f6