Good points, Tom! While I am not suggesting a Ranger Tug as a year-round live aboard boat, the ones that are ordered locally almost always have Webasto diesel furnaces installed. I even had a heat outlet added to my (fully enclosed) cockpit!
As regards speed, yes, I see a lot of Ranger Tugs zooming around. I think many are the new designs with outboards, and a hull optimized for planing. The original designs, and still the R-29 and R-31, were semi displacement, not planing hulls. I and a lot of other owners operated them at tug speed (below hull speed) to optimize fuel economy :-)
For living aboard, I think your suggestions for Nordic or American Tugs are just right!
42 Nordic Tug Spinner