wrote: Does the existing door have any historical value?
If it does, it might be worth rehabbing it.
If it doesn't, it might be worth a visit to an architectural salvage company to look for a door appropriate to the period and style of the house.
Have you thought about putting a storm door over the existing door? You can get a thin frame door with store door insulating glass that will allow the old door to show while giving you some of the energy savings you are looking for.
Rehabbing old doors means taking them entirely apart, cleaning up the glue lines of the joinery, repairing splits and structural flaws, etc. If it is a good old door, the exercise can be an education.
And it sounds like a fun project too!