Don’t buy that 13 SEER replacement unit you were thinking about. The 2015 New York State (and International) Energy Conservation Construction Code has 2016 efficiency standard increases built in. Minimum efficiency standard increases kicked in on 1/1/2016. You can’t replace that rooftop package unit with a bad heat exchanger with a 13 SEER anymore. As of 1/1/2016 it has to be 14 SEER.
You can view all of the minimum efficiency increases for commercial heating and air conditioning equipment here at the International Code Council. You can find the new residential heating and air conditioning standards here. Most states have now adopted the International Energy Conservation Code.