What's the seller's motivation to sell it below market enough so that it's attractive to an investor?

@Marc DuLaneyI would look up NARPM and start with them..Below are some questions I would think would be a good starting point for you to see who really treats their company like a business or a hobby.Questions to Ask prospective management companiesWhat are your average days on market for vacant homes?

In areas where folks pay 100% cash, who cares what the GRM is, if they know, or care to know what it is.When I started investing in real estate in the early 80's, I bought properties with a GRM of 6 or below.

Or go directly to the tax assessors office and have them filter out a list of multis in the area, then skiptrace the owners personal information to cold call or direct mail. 

@Andrew ReidOne of the main filter systems I used when starting out is to go to a landlord friendly state.  

Smoke detectors, carbon monoxide detectors, furnace filter, under sinks, toilets, windows, roof and gutters.

For instance are home buyers willing to pay more for a house like that knowing their maintenance costs will be lower and return on investment higher (generally)?

You might take over furnace filters if you plan to supply those. 

When I check for prices, typically they are below previous peaks.

That will give you a good idea of the market rent.I know that where I live in Kent, rentals below a certain price point get snapped up within hours.