I have it in my head that i'll use supported rails now...I know that its worth the extra pennies. Looking at chucking a load of old hobby bits on ebay to help with the funding.

I had previously been planning on using steel box - I think we exchanged messages about it ages ago.
I like the convenience that ally profile offers - I also have concerns about getting a frame built with steel box square...although since looking at the supported rails I did think it could be easier to attach them to a frame made from box rather than profile.

On the subject of rails, for a 2'x4' machine, what size rails would you recommend?
I've just been reading a load of build logs & have seen mention that 20mm is overkill for a machine this size & that 16mm may be a better fit...but then there were comments about the bearing size being harder to work with - found it all a little less than clear :-p