I would use the resources like yellowpages, google reviews, ask at a real estate investor meeting or maybe sign up for Angie's list and find the best HVAC company and installer who, stands behind the product, is willing to educate you on the right equipment and provide you a fair quote.Your investment strategy, and what the standard is in your area will dictate the price.I would look for the Highest seer, with a good warranty 10 compressor, 10 year parts, look for one with quiet technology (compressor sound blanket, motor mounts, Air handler blower motor, air flow designed for quietness) and ease of service for the air filter, air handler coil cleaning, overflow float switch, and access or instruction on drip pan tablets/liquid and drain line cleaning.What I look for in a HVAC installer is someone who is proficient and not on the phone or looking through the manual a lot.  

You can cut vents into the walls and the natural airflow should be sufficient to dry it out.

If it is not changed it will restrict the air flow and cause the furnace to work harder which will eventually require additional service=$$$.

It could be a lack of proper air flow around the coils, the condensate not draining away properly or draining too slow, the refrigerant could be too low or leaking, the air filter could be clogged, a damaged or broken fan, etc.

A dirty filter restricts airflow and strains the equipment.If you want your tenants to change the filters, you're going to have to supply them.  

This is to maintain proper airflow.

If its the attic, your insulation needs to support it, if not, or up to date with new units is the coils,  by virtue of how fans work,  the coils will not get sufficient air flowing across them.....

These windows open as a hopper fo ventilation (10 - 15 degrees) or swing open into the room (much like a door) for egress, washing, or when you simply want more airflow.  

I found magazines and shoved plastic and paper bags inside. the old tenant from the previous landlord I guess did that and I never thought about checking in there to begin with when I realized the air flow was bad.

Gas & electric are both common here - the only reason they catch on fire is because idiots let the lint accumulate & block the air flow, not because of the fuel source.If you go with electric, be sure you have a dedicated circuit run for it - that's another source for horror stories.