Whether window, split, or central, plan on specific periodic maintenance/cleaning to keep them operating at their best, and prevent breakdowns in the worst part of the season.A final consideration regarding window units, you need to ensure they are properly and securely installed and condensate drains are routed away from the structure.

The next thing that happened was over Memorial Weekend, received a call saying the AC Condenser stopped working.

Low temperature hot water baseboards (140F supply) with a condensing boiler and split systems for A/C would probably be the best bet for balancing having the tenant pay for some utilities and not having their bill be super high.  

HVAC scope of work back in 2017,1) Condensate line checked for clearance 2) Ducts not smashed 3) AC’s exterior lines insulated & cage installedThe AC unit was not new and as of today is about 12 years old.

If a condenser blows up, my guy puts in another used one for like $500...labor and materials.

If the mold looks like its growing from the bottom up, or from a spot and outwards, and if there is any soft drywall, it probably is from a leak.However, I have seen cluttered houses get surface mold on exterior walls because of bad ventilation, the clutter doesn't let air flow and some walls (especially exterior facing northeast, where it gets no sun) can be cold enough to encourage condensation.

I use a combo washer/dryer unit that is electric and requires no venting (everything is condensed to water and drained out through regular plumbing).

Each unit would need its own outdoor condenser.

There really isn't a huge difference between windows, doors, HVAC, mechanics, electrical, or plumbing from homes ranging from 900-1400 square feet, these don't really multiply until the home gets big enough that you need 2 electric boxes or 2 condensers/furnaces, etc.

These are AC/Heat electric powered exterior condensers with interior wall units.