I came to pretty much the same conclusion about using steel, I can upgrade something like a spindle or motor at a later fairly easily to improve the machine but if the frame moves around like a jelly then the whole thing would have to go in the bin and get rebuilt from scratch.

At the moment I'm planning to hold thep pieces of steel together with brackets made from steel angle and bolts so that I can adjust it fairly easily, because of the problems welding seems to cause I'm also considering joining some bits with a metal epoxy. If you need to buy a welder just for this job (the really cheap mig welders won't do 3mm/4mm box properly) the epoxy route would work out cheaper and easier than learning how how to weld on a frame that needs precision work from the get go.

This guy has a lot of useful info for how to approach frame building, while spending £500 on a perfect straight edge to use for alignment may be out of my league there is some good stuff that can be applied on a lower budget.

Machine frame - MadVac CNC