I also worry if I answer him, it will open up a dialogue about "why" and I really don't want to get into a protracted debate about it. 

As part of your screening you can ask the applicant how many people will be living at the property....If there are applicants that say just her and 2 kids and you have other applicants that say her and her 4 kids and their two puppies and maybe her man.....then common sense tells you who to continue the dialogue with because you know which families will be less wear and tear on your property.

I do not engage in any dialogue on FB messenger after the canned response.  

I am early enough on my adventure that I don't have a lender yet :) (or a Realtor although I started dialogue with one).I am open to recommendations if you or anyone can offer.

The dialogue can be different to your natural flow.

I would do your best to go to the property and try and start a dialogue with them.

Typically I ask them what their budget is and do an on-site estimate but I'd like to get the dialogue started before the initial meeting....

In your case however, it seems they are not distressed homeowners but just absentee homeowners so the dialogue might be more inline with dealing with commercial landlords that really do not need to sell in a hurry and are not in a financial position where they might lose their home.

The numbers are a bit askewed, but another idea is to reach out to the owner and start a dialogue to assess where the numbers are currently and what his/her motivation is to sell. 

Open the dialogue and don't talk about specifics.