here what I know,
as mentioned above, insulation is extremely important. The second is your windows rating. If you're not interested in replacing your windows, never mind. But there are two ratings, the U factor and the SHGC, and they make a significant contribution to you houses overall efficiency.
Next is your condensers SEER factor. Bigger the better. To get a higher seer you need to go to a 2 stage pump and this is where the price takes off. So that's a judgement call, is the savings in power going to justify the added cost.
I forget how the tonnage rating came to be, but it works out to about 20,000 btu per ton. My understanding is, bigger is not better, it needs to be matched to the heat loss of the house. That depends on materials, roofing, siding, insulation. It's not all that hard to calculate. (look online)
Lastly, HVAC contractors are the most crooked of all the trades. Don't trust them as far as you can spit. Only problem, you will need one to do the install. Like Gregsy said, you CANNOT buy a condenser without a license. (I tried, almost got away with it.)