Re: Antique house - replacing furnace
A Geo-thermal system is a heat pump that is supplied with about 52% degree water regardless of the outside temp. Sizing the system to your house may not be cost effective.
The hydronic system you describe has a serious flaw, any time you do a heat transfer from one medium to another you have the cost of the loss. No system is 100% efficient.
You already have duct work and you want AC. When we changed from oil to propane we lost the smell of the oil, soot on the roof, and half the heating cost. If it were me I would install an 80% efficient unit. The cost savings in fuel for a 90+ unit is offset by initial cost and increased maintenance costs. You should be able to get a humidifier that will work with your water.
Be sure you live your life, because you are a long time dead.-Scottish Proverb