So our forced air gas heats the old part of the house downstairs, and the mini-split heats and cools the well-insulated upstairs. 

Those old govt homes are big and not very well insulated.

-tickets to a temperance event-old coins-racist cartoons from old newspaper (used for insulation)

Did that help contribute to the lower cost or is it typical in the area to have the pipes run outside with insulation?

I had a few companies come by and give me quotes on insulating, drywalling, and painting.

Depends on the insulation, size of the house, how you set the thermostat.

Currently, I have a few smaller projects on a couple of houses: carpeting/tiling floors, painting walls that contain LED paint, installing A/C and heating system, installing light fixtures, installing appliances, making sure electrical wiring is safe, installing recessed lighting, insulating/waterproofing medium size room in the basement.

Need to make sure insulation is solid in attic, and your CFM and btu/hr is high enough to handle the head pressure it has to supply to push that air up that much vertically. 

There is no insulation between properties in these old buildings.

I have seen people miss many obvious things (drywall, insulation, HVAC, etc) and they were costly lessons.