As I'll bet you already know, There are endless people here wanting short term (6 months or less) while they wait to find their dream home or wait for their home to be built. 

My AC guys calls would start out as home warranty calls, but the home warranty company would either delay endlessly or deny the claim, and the home owner will be stuck paying the bill. 

This is very important for those of us who post endlessly to this forum into the wee hours of the night :). 

BiggerPockets offers an endless amount, there are tons of other resources out there as well, and I think that just continuing to dive down the rabbit hole will help you frame your thoughts. 2) After a few months and once you have accumulated your cash reserve, clear prioritization of the many ideas that are bubbling up will emerge.

And I don't have an endless amount of reserves.  

step 1: talk to him. step 2: listen step 3: come back and inform us what he says.we can play hypotheticals all day. up till the point you ask how he feels about the offer, it's an endless cycle of if's. call him, say you were curious to hear his thoughts on owner financing, when he asks what you mean, share these details. if he is reluctant, spell out the pros of doing it and then the cons.

As others have said, there are endless options out there. 

Old Homes Pros: Tend to be in better locations closer to amenities and main attractions, more likely to be walkable, often have larger lots, houses and surrounding area have more "character", sometimes they have features like old growth wood and other elements that add value and longevity, no HOAs; Cons: Near endless potential for expensive issues of every type because of the impacts of time, neglect and old building materials and methods, require more work and money to make them modern and appealing, can be tricky to financeNew Homes Pros: Far less maintenance and potential for issues and may still be under the builder warranty if issues do come up, modern design elements and fixtures, often in good well-funded school districts; Cons: Homes and neighbrorhoods have a manufactured feel and less character, lots tend to be smaller because developers want to pack in as many units as possible (especially in high $$$ markets), tend to not be walkable because the areas adjacent to the best attractions/amenities have been built on already, sometimes built with low end materials, HOAs restricting what you are allowed to do to your property 

These people will have to share a house together, and you don't want tenants who are messy, loud, inconsiderate of others, or immature--these types of tenants will just cause you endless headaches (and liability) in a rent by the room property.

Even if couple owners would not sell, your returns DO NOT depend on them selling or not.It's not like you are developing land.And then, you can also flip the units, if the debt needs to be repayed faster, and it wouldnt look out of whack, so overall, endless opportunities here.