Do you have access to natural gas? That is probably the your best option for heating.

The problem with geothermal is the initial installation costs are high even with the 30% tax credit. Another alternative is an air sourced heat pump. The inverter type such as the Carrier Greenspeed can get close to geothermal performance at about half the price. It is worth investigating.

no NG in area. Probably would put propane in as backup and for future kitchen upgrade.

Once you move in you can begin getting quotes. Ideally you would want to do the work in September or October when things slow down for the HVAC contractors.

Make sure you are sitting down when you get the geothermal quote. It gets expensive when you have to install two independent HVAC systems.

Seconded on the importance of getting quotes/installs in the off-season. The quote/install quality can often increase substantially. In the off-season, way more attention to detail is paid when things aren't so hurried or rushed. Some will also give discount if paying in all cash, sometimes as much as 5-10%.

Wait and see how the current system keeps up. Assuming that the heat and cooling are operating nominally, you can know what size system will be needed to replace it. You'll have to pay attention to cycle times and humidity levels to really know if the current system is sized properly. Better to make the observations with the old system than find out that the new system is sized wrong.