Today our HVAC contractors began installing the furnace and a/c unit! They should finish with the ductwork tomorrow.
We ended up going with a Carrier Infinity 96% efficient furnace (so tiny - like 1/2 the size of our awesome 50's model)
and Carrier 17 SEER A/C (holy cow that's big! We're planning to put a deck on the back of the house, so we werent' too concerned with placement at this point - it'll have to be moved eventually).
We also added a whole-house air cleaner (HEPA) by Aprilaire. Since the house was not built with A/C, they needed to add a couple of cold air return vents on the 2nd floor, as well as a couple of new supply registers.
My mother in law was wearing gloves in the house today, so I'm glad this is getting done! Apparently not a moment too soon.