I built from a basic kit (Avid Pro 4 x 2, for chair work), but modified much of it to accommodate specific functions. Got backed into a corner along the way and ended up building my own control. My original intent was to build a fairly simple machine and then make stuff on it. I had prior experience doing cnc work, but now retired, I wanted to keep my hand in and make stuff. The kit seemed to fit the bill while allowing for future expansion

The notion of building 2 different capabilities into one machine may have some merit as long as you separate work envelopes. Beyond that, all the usual concerns should be part of your deliberations. Things like vibration/harmonics, frame rigidity, accuracy, etc. Resale value and ease of moving the machine will have a bearing as well. Something that jumped out when I first saw your post was the length of gantry. It will be a challenge to keep the weight down to pare drive line expense while still keeping harmonics and vibration out of it. All doable, but difficult to get right first time out.