Hi everyone, My wife and I own A Early 1800's stone farmhouse. We are considering installing A geothermal system to replace our current propane system. I would like to find info from anyone who has had experience retrofitting A home like mine. Any advice and opinions would be appreciated.

Since you already have a forced air system, retrofitting is pretty straight forward. There will always be challenges but nothing that hasn't been done before.
The main thing is to make sure that the right sized geo system is proposed.

How is the house insulated? Hopefully foam. If none, then get foam insulation installed. Otherwise, your heat load is going to be tremendous. Of course, that depends on where you live...