Being that i have done a few design jobs for geothermal systems on commercial buildings, i can tell you a few things.
also, try to get a hold of Greg Wells of Jackson Geothermal. 419-566-1095
3401 St. Rt. 13 South
Mansfield Ohio 44904
1. its is not cheap. The cost of the bore holes is high. Can be up to 1000 dollars per hole depending on your location. We had a site that had 40 holes.
2. There are actually two ways to do the loops, one is vertical the other is horizontally. odds are you don't have the means to do horizontal (shear space) to get enough loading.
3. The furnace has to be replaced. Anything that uses the existing furnace is either crap or not effective.
4. You will still have an electric aux. heater.