Rails on my build are 5 feet. I have no noticeable issues with sagging even with the DW660 on. Accurracy is spot on.
Changing tools is as simple as a few screws… Although I need to come up with something better for wire management… Quick connects or something.
As for height I would recommend giving yourself ~4 inches of adjustment. If you are engraving something with depth. I have been engraving mirrors and while most frames are pretty thin there are some that are quite thick. The added height should not really matter even on the milling side, you can always raise the work a bit.