My stepmother has a Mitsubishi heat pump that I’ve seen work great even at -5 and windy .

@Gareth Fisher  I see a lot of Hitachi and Mitsubishi out there, but I can't really vouch for them as I have not installed them in my own home yet.

We use Mitsubishi hyper heat, cause they work down to -13.

About 4 years ago when I was living in Uruguay, we had a Mitsubishi Galant.

Gree units are less expensive (initial capital layout) than, Mitsubishi, Fujitsu (20-25%) or even Daikin.  

Slim Mitsubishi air unit which is a lot less than central air and can solve their problem for the few summer months that the temps will be uncomfortable.

I've had 18 mini splits installed in my previous residents, rentals & commercial properties: Sanyo, Union Air, Fujitsu, Mitsubishi, Panasonic, Comfort Aire, Daikin, Soleus Air.  

If they plan on renewing they get the convenience of new AC if not you can install the system during your vacancy and your new tenants can just sign a lease for a higher rent (for the AC upgrade) assuming your market commands it.IDK what how big your units are or the building but if they are small enough and space is limited you might consider a Mitsubishi split.

We installed a Mitsubishi single head mini-split in our family room about 5 years ago.

My Mitsubishi mini splits that are 15+ years old have required little to no repairs, and parts are generally available, while some off-brands went out within 2-5 years...