Upgrade or stay put

[beenthere]

15 higher efficiency, uses less electric per BTU then 13.
16 also higher efficiency but 2 stage for better comfort.

[Taylormade2]

The cost of just adding the additional supplies and return is about 1/10th of the cost of a new system.

According to the person who gave me the estimate, he states that he can't guarantee the system will cool properly if the duct work alone is done. This individual may be honest or perhaps he just wants to make a sale.

How do I know if the current 10 SEER , 3.5ton Rheem unit can handle 2 additional supplies and the installation of a return?

[beenthere]

Well, if he would have checked how much air your system is moving he would know.

Since he didn't check, he's just guessing, and hoping you'll decide to get a bigger A/C.