Arc welder purchase.

[birchy]

Personally, I wouldn't touch anything Chinese. Actually, that's a lie because most kit is made in China nowadays. What I mean is...I wouldn't touch anything unbranded. It's all about guarantees, spare parts and after market support. You'll get no guarantee or warranty with that machine. If it goes wrong, you have to throw it away and buy another. 200A will comfortably weld 12mm thick plate. You would use around 100A and 2.5mm rods for the box section you suggest, so anything 150A+ will be suitable, however I suggest you don't go less than 180A as you will inevitably get some thicker jobs in the future.

The inverter sets are nice to weld with...if you can afford £250ish. THESE are top of the range and will never let you down. Oxford are another good make...especially the old bomb-proof, green oil-cooled sets like THESE. Don't touch Clarke or Silverline, they're no better than that unbranded chinese machine. SIP are made in china but have decent guarantees so if it goes wrong, you can take it back. GYS are French and of decent quality. We have a GYS TIG set at work and it the canine's wotsits.

On a tight budget, I'd be looking at good quality second hand kit rather than low quality new. You could more than likely buy second hand, use it for your job and resell at what you paid.

Or you could just hire or borrow one to do the job.