I use a cheap angle grinder stand from Lidl, and a 115mm angle grinder to go with it. You can pick them up pretty cheaply. It's not perfect but neither's my welding! You might save enough to buy the tools by finding a cheaper metal supplier, and having an angle grinder to hand anyway will be useful during the build - deburring, trimming to fit, etc, especially if you have any diagonal braces. My local supplier will cut but doesn't guarantee to better than 2mm although the pieces I did have cut were pretty close. I bought from Austen Knapman; they're in Devon so might not be too useful but it's an alternative place to get pricing. I've also used Metals4U in the past but I think they only sell by the metre.