The main things you want to look for or avoid in the area, in my opinion, is going to be Flood Zones, moisture issues in the crawl space (it's very humid), and maybe knob and tube wiring if it was built before the 1940s. 

The other thing that may apply to you in FL, that sometimes happens in CA is a property not having an AC system if it is close to the coast, probably less relevant given the humidity in FL but I thought it was worth mentioning.

For example, if I put a device that monitors humidity above the drywall in the ceiling, would that be of any use? 

Note, this is south-east Pennsylvania so the summers can get hot and humid in my opinion as an Icelander but nothing like the more southern US states.I'm willing to explore all options and am literally at the beginning of my RE investing career, any and all help and comments are greatly appreciated!

As for the humidity, bring it.  

These items can last for a very, very long time if moisture, humidity, and bugs are kept away.I specialize in pre-1950s houses and have one from the 1910's. 

Good evening all,My rental unit has recently had little bugs in it, they’re harmless they aren’t cockroaches, they come from humidity in bathrooms and laundry rooms.

You will need some space for the second unit, preferably close to the first unit, so that tapping in and re-routing the ductwork is minimal.Alternatively, dedicate the existing HVAC to the larger apartment and install a mini-split system for the other.One of the issues with both solutions is that the existing HVAC will be oversized for just the single unit, which can lead to issues with moisture removal during humid periods, but in my case, this has worked fine for several years now.

There was no water leakage in the basement and no humidity at all.

You need the exhaust to remove the humidity from the shower area.