If your furnace has uninsulated ductwork (most heat only systems are) your looking at replacing all of it. You don't give house size or age so I'm going to make some assumptions (I'm in GA, so they will be sized for a hot/humid climate).
Lets assume 2400 sqft - 1200 per floor. Off the cuff I'm gonna say cooling needed is 4 ton. Could be a bit undersized, but it'll help with humidity.
You're looking at ~2,000 to 6,000 for the equipment, depending on SEER rating and manufacture. I like Goodman/Amana - the 14seer distinction series is now coming w/a lifetime warranty on the condenser and furnace.
Labor is going to be the big variable - that's probably going to be another $2,000~$5000 depending on access, line set routing, amount of ductwork, etc. From a few hundred miles away I'd probably put most of the estimates your going to get from $7,500 to $14,000 installed.
Get multiple estimates - compare equipment warranty and how long the contractor has been in business. Since you do not have A/C now, make sure anyone quoting they system sizes it by doing a load calc. The tonnage you need will depend heavily on wall construction, insulation, siding, the number and type of windows (and which way they face).
Under cooled isn't nearly as big an issue as over cooled. An over cooled space will feel cold & clammy. Under cooled will be fine most of the time, but will not be able to keep up on unusually warm days.
Good luck and keep us posted on any quotes you get. Its interesting to see the range of prices in different parts of the country.