Do you have a full basement and a forced air furnace or is it hot water heat with radiators?
A proper heat load calculation which you should do yourself:Load Calculator
is necessary to get the proper sized unit. Lots of contractors don't do them or properly. Getting 4-5 quotes from long term reputable (check BBB) contractors is recommended as well as talking to your neighbors, co-workers, friends to see if they can recommend someone is the best way to go. Check back here once you have more details, we don't do pricing as it varies all over the country according to cost of living etc.