A post-office box is not sufficient. 

If it's a 3-4 unit property, FHA requires a self-sufficiency test that the property needs to pass. 

The attorney I did speak with advised that it may not be sufficient just to have “back payment rent” as a reason to evict these tenants.

When we have sufficient funds in any of these accounts, we tap the excess to pay down debt or purchase more real estate!

With a crawlspace there has to be sufficient ventilation for moisture to be cleared out from under the house (much like attic ventilation).

We would be financing with 20% down and have great credit and sufficient financial assets.  

We think it’s important as it allows us to check for Non-Sufficient Funds (NSF) issues and to determine if an applicant is living paycheck-to-paycheck.

You can also do that with a 3.5% FHA loan, but an FHA loan will also require the property to meet a "self-sufficiency test," whereas the 5% conventional option does not carry that requirement. 

More than 50% and they can say its sufficient?

Any form of rehab loan there's more due diligence involved and more processes to go through to make sure that the loan you're getting is sufficient for the type of rehab you're going for.