Hi TE - You need a 600W spindle (prefer 800W 400W in timber will stall too easily) , you need to make the bearings captive to the rails in all directions. By the time you buy all those bearings and bolts you will spend more money then buying round rails and cars. Yes welding will distort your tubes way more than 1/64" ( 0.4mm) 18" of Z is 457mm which is huge. Need a very stiff machine to handle that. Commercial machines are about 6" for a good reason. I'd build a plywood machine before going down this path. Less then $1000USD is doable, 12mm wide belt OK but for 8ft machine I'd go to the next size. I use 16mm wide on mine could be a little stiffer. Your design will work but med term you will be disappointed with the tubing as rails and skate bearings for the carriage, too many bits to come out of whack which they do often. Rethink those to supported round rails.
If you use the high rail/low rail gantry you will have to connect them somehow to stop them wobbling. Good luck and keep designing.. Peter