I recommend you look at a heat pump system that meets the tax credit criteria.

Good brands are York, Trane, Lennox, and Carrier.

That systems will have an overall SEER of around 15.

Your energy savings on your heating bills with a heat pump vs AC will be worth the additional cost that a heat pump system will run.

Your "sweet spot" for savings in energy cost and comfort come at this level of equipment, IMO.

soonermom

first off,I like HPs as well-for the right application. HPs are not for everyone or every application. There are many factors to consider for HPs and those include location/winter climate, comfort, home's envelope/insulation properties, and electric rate vs other alternative heating fuels.

one system or two for such a large home?

I would think a 95% eff furnace paired with at least a 15 SEER AC or HP.

any hot/cold spots within home need to be evaluated by dealer and recommendations made to improve.

brands to consider are Carrier/Bryant, Trane/AmStd, Rheem.

if you have long hard winters then perhaps one of the new modulating furnaces like York. I do recommend any furnace to have the VS blower.

what size HVAC are you replacing?

request a load calculation from quoting dealers for both cooling cooling and heating to determine correct sizing.

IMO
Good Luck!