What you are referring to is a "Tube Notcher", lots of different types and quality and prices around, give it a google and see what comes up. Got mine off of fleabay for just under £90,uses standard hole cutters like "Starret", don't use "Bosch" quick change ones as they wooble about. Had to fettle it to cut centrally and the mounting arrangement had to be modified to get it to fit onto the pillar drill. This particular one needs about 14" between the chuck and whatever you mount it on. Probably going to re-mount it horizontally on the bench and use a hand power drill. I have seen some home made ones on "Mig Welding- The DIY Guide", useful site if you are going to be building a frame (spaceframe?). I would think that you will need a roller type bender for the tube, the Clarke hydraulic benders will probably deform the tube, and CDS tube is hard! If I were building a frame I'd use TIG to stick it together.