we did this last summer. we should be zero net energy this year. our gas furnace was shot and we needed to have our roof done. the roofer offered to remove the chimney so I went for it, junked the furnace and the two fire place! I did a lot of research. geo-exchange/geothermal is hardly competitive with the latest super efficient air sourced heat pumps. Mitsubishi has the best, from what I can tell. we opted for a 3 ton Pumi unit. the latest ones can go sub zero without the need for supplemental heat. we left our ducting in place and added an air handler rather than adding a bunch of head units, basically one would have been needed for every zone in our house and our house is long and narrow, so not cost effective. the air handler went where our old furnace was and transfers the energy back and forth using a blower fan to circulate the air in our home. it's quiet, super energy efficient and runs on pure electricity which is net solar for us. it's got a rather different feel than our gas furnace blower, it's more gentle and can hardly be heard, it's kind of on at a low idle most of the time. we also opted to add on an electrostatic air purification system with ultra violet, mechanical and activated carbon filtration... the air quality in our home is so much better now! If you are going with old ducting, I highly recommend getting some industrial quality mastic and sealing up the ducts in the crawl spaces. our 60 year old system had come completely disconnected in spots!
As for insulation, the best way to do it for old houses is to have someone come in and do blow in insulation. there are zero or low VOC versions that work great. I have gotten handy at cutting slabs of poli iso insulation on a table saw to go in between the studs and am going through the pains taking process of using cans of blow in insulation to seal the edges. I don't know how far I will get with this, but I figure I'd at least do the basement where getting to the stud spaces on the exterior walls is easy.
