We're tearing down part of our house (about 1/2) and will be redoing all the utilities. The question is... if we use geothermal radiant heating in the newer part of a home (basement and two floors) and put a pellet stove in the old part of the house (250 year old field stone) do we even need new air duct work? Is this feasible? (house will be 2600 sq ft. total). I'm wondering what I should tell prospective HVAC contractors.

Yes it's okay if you don't need or want cooling by the Geothermal unit!