Capacity question

I have a single story ranch where I am having a central AC installed. The house has forced air heating so there is existing ductwork. I had an installer here for a quote and he told me that the area is about 1500 sq. ft and he would put a 2.5 tonne unit. I have a family room with no ductwork and a separate heating unit. I would have a window AC in that room. The area for that room is not included in the 1500 sq ft mentioned above. This family room is connected to the main house through the dining area so there is no door.
I want to know if the size of the unit is correct? He said that if he puts a 3 tonne unit in, it might have problem pushing the air through the existing ductwork which is not top quality. So it might cause more problems than a 2.5 tonne unit. Also these parts of northeast don't need that much cooling so it should be sufficient.
He offered me a 10 SEER unit (brand new, Rheem) with a 5 year warranty. Compared to the 13 SEER unit (also Rheem) with a 10 year warranty (on the condenser, for both), this one costs me about $450 less. I think I would be ok with a little less efficient unit and lesser warranty but is there something else on the 10 SEER unit that I might want to take into account now?
Finally, is there any other question I should be asking the installer to verify the quality of the install? I have gone through some posts in this newsgroup and I understand the quality of the installation is as important as the quality of the unit.
Thanks in advance for your time.
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snipped-for-privacy@gmail.com wrote:

Considering the added 5-years of warranty I might go for the 13-SEER for a mere 450 difference. The 13-SEER needs to have the Scroll compressor and get the TXV refrigerant metering system!
The savings between the 10 & 13-SEER at present electric rates in your climate won't be much, however, rates could go up.
Have a manual J heat-gain heat-loss calc done, and then do all you can to reduce the load, then refigure the calc to a smaller tonnage and less heat required during the longer heating season. Have the duct system thoroughly checked & tested for leaks and correct CFM capacity, and also unwanted Air infiltration from the attic and other places is a huge factor to address! Those are the areas that must be addressed first, then proper equipment sizing and a quality installation! Where you live, SEER differential is a minor factor!
- udarrell - Darrell
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Air Conditioning\'s Affordable Path to the "Human Comfort Zone Goal"
http://www.udarrell.com/air-conditioning-total-heat-enthalpy-latent-heat.html
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