Hipsters and Millennials seem to like them a lot.In my opinion, the easiest houses to own, rent and keep in good repair are 3 bed, 2 bath, 2 car garage ranch style homes built after 1978 (to avoid lead paint issues) with a basement or at least a 4 foot crawl space that allows easy access to plumbing and HVAC duct work and a fenced yard. 

Another trick I have done in the past with homes with the old high ceilings was to run ductwork up from the basement and strap the ducts to the ceiling.

The service contract is a one year contract covering a multitude of systems and appliances needed for everyday life.Here are a few of the key things to remember:•No home inspection is required as a condition to purchase*•The contract covers certain household system components and appliances regardless of age, make, or model*•You pay a trade service fee for each occurrence, regardless of the actual cost to repair or replace the broken item(s)*Our Basic coverage includes:Heating Systems Plumbing Systems Electrical Systems Water Heater Oven/Range/Cooktop Dishwasher Built-in-Microwave Garbage Disposal Exhaust Fan Ceiling Fans Plumbing Stoppages Ductwork Or you can expand your coverage to include:Central A/C Refrigerator Clothes Washer Clothes Dryer Garage Door Opener Well Pump Pool Spa Several afforadable plans are available to fit your home and budget needs.

It will go everywhere air will go - including inside closets, hard to reach spots, inside the duct work etc. 

What typically has happened is that since the homes are very old, the ductwork was designed for heat only.

The decision is to either patch the current ductwork or replace it with brand new and more efficient work.

I can relate with your comment about animals getting into the duct work for central air as well.

Speaking from experience I try to get deals where I can... say rent is 600 a month retail hvac unit duct work is 5k ok after taxes insurance that means the tenant has paid a year long not missed a payment your still in the red... that’s why I pay cash.

But they are also ugly, noisy, and expensive to run, so it depends a lot on the class of property and what your goals for the property are in terms of repositioning.It isn't always as simple as just slapping in a central unit; Electrical panel and/or service upgrades are sometimes needed, and you need attic or crawl access for duct work and returns, as well as a place for the air handler to go (which often means giving up a closet). 

Running HVAC trunk lines to the old ductwork was another fun issue.I pulled the permits, had the inspections (prior to & after drywall) & passed.