Sounds good to me.
I'll go watch the videos on the rail comparisons.

re:welding, my only concern is steel is very difficult to weld and maintain high tolerances, for instance creating a right angle between two pieces of steel, after you have prepped and clamped into position, then welded and cooled down, you might end up with 88 degrees instead of 90, most things that doesnt matter, but if you then wanted a true 90 deg out of it you'd have to machine faces.

I think that is why Ali extrusions are desirable as they are made to high tolerance and obviously adjustable to square up.

Maybe there are welded CNC machines on here, I'll take a look out of interest. But unless there are huge advantages I'd be prefer to bolt together, and maybe weld the odd bracket if needed.