Considering your planned use, I would tend to go for an arc welder, the biggest I could run. I remember welding 1/4" steel(or was it 6mm ?) using a 3 phase arc welder set to about 150A, and using 3.25mm rods. It was a simple job, 3 passes, root, filling and caping. If I had to do that using a 240V Mig welder I would use a lot more wire, and the job wouldn't be as good, as it wouldn't adequately melt the parent metal. I don't think Migs put out as much heat as a decent arc welder. However most people, myself included, buy them for car repairs, and they excell at this. I once tried to weld cills on my Avenger using a SIP 140 and 1.6mm rods !

. As I said underseal convers a multitude of sins!.
If I was rich, I would prob go for one of these http://www.machinemart.co.uk/shop/product/details/arc-tig-155-power-inverter as not only could you Arc weld, you could TIG if required, which is really fun. Thats how you get really well controlled, quality welds.
Ken