We have a heating unit in the garage that heats our 3 bedroom home is it possible to add an a coil so we could have AC, roughly how difficult and exspenxsive would that be, any advice would be greatly appreciated.

Generally yes. There are way too many variables. Have a Manual J calculation performed to size unit and get 3 estimates.

The SEER rating determines operating cost.

I assume you have a ducted heating system. If so, you can add a cooling coil into the duct with a direct expansion (DX) condenser. Daikin make a very good model. You could actually get a heat pump version and use the same system for heating (saving your furnace).

Relatively easy to do but as I'm in the UK I don't know about the cost - maybe $1,000 to 1,500 installed