You said you are doing your own load calculation.. What is the indoor design temp that you are using for your calc.. Just be aware that A/C manufacturers rate their units at 80degrees indoor temp.. Also a 2ton system may not actually be rated at 24k btu..
I am using the default temp from the HVAC-Calc program of 91 deg. for summer and -1 for winter. Should I trust this or change them. Just so you know the house has an extremely tight envelope. R-13 with a 1" polyiso ext. with house wrap and R-38 in the attic. Double pane, low-e windows and the ducts are run inside the heated space.
seee my previous message to answer your question on the coil. coils typically don't cost over $200. will this make or break you?
I suggest calling a contractor and asking for a heat load calc and installation of a new unit and matched coil. If you give him a copy of the plans to your house, his supply company will do the calc for him for free.
then he will give you prices for each of your options as far as the seer goes. you can decide what efficiency works with your budget. "less efficient means more energy consumption, it has nothing to do with comfort when properly sized."
KEEP IT SIMPLE, don't stress over a $200 coil when it could cost you quite a bit more if the installer doesn't properly adjust the very touchy txv valve to obtain proper superheat.
As an example, that coil with a Rheem RAKB-024 10 SEER rates to 11.00 SEER. Not too shabby. Another match is the builder 12AJB30 for 12.00 SEER so it can be used on 2.5 ton. It's rated with the RAMC-024 and RAMC-030 to get 12 SEER. With the new RAND-024 it gets 13.00 SEER.
If you don't want to use Rheem, your dealer can check the match for his brand of outdoor units.