Hello ArthritisDoc and Welcome to the Forum:
You could check with Consumer's Report to find fair, unbiased ratings. I think the 13 seer would be sufficient; the 18 seer would not pay for the difference in its econimic life.
Avoid service contracts like the plague, they are only good for the seller and if they are selling junk, I don't want their equipment or service.
Electronic air filters are good but there are too many high efficency filters out there without the electronic hassle. I have a Research Products corp Space Guard. It is a big, fanfold filter with a low micron rating (catches all the little stuff along with the big stuff). I am well pleased with it.
Yes, your ducts should be checked (possibly cleaned) checked for proper insulation and updated. Heat pumps depend on lots of air movement and leaks defeat that.
My heat pump is a Geo-Thermal unit; if you want to go to the best it probably won't cost any more than the 18 seer and the geo will pay for itself.