I got an arc welder from Aldi which was very cheap. Think it was £30, maybe £35 and claims to be 160A.
I've only used it to weld my router frame and practice, but it seemed fine to me. The thermal cutout is a bit annoying, but didn't activate too often when welding 3-4mm.
If you're going to use it a lot I wouldn't reccomend it, but for one off jobs it's good considering the price. I reckon if I added a bigger fan it would be fine...

This is it, a lot more money here though:
http://www.ebay.co.uk/itm/Silverline...#ht_500wt_1054
There seems to be a lot on eBay...second hand?

I think Chip will be able to give you better advice on welding...