We have one vacancy that we will probably leave empty for now unless a really good applicant turns up. 

Post the eviction notice per your lease and applicable law.

In my opinion, based on my limited experience both on the ground and theoretical, this property would cost an investor like me money every year, or perhaps show a false income before needing to take care of serious CAPEX issues, vacancies, or turnover costs.If I can put a real world application in place, which is where I think you are headed, let's say you cash flow about $1500 per month, or 18,000 per year.

However, the SBA Emergency Injury Disaster Assistance Loan (EIDL) loan applications I've seen talk about covering mortgage payments, which I believe will be your main qualifier.

Based on the size of the home, this definitely feels doable if you're sticking with more of a siding/paneling application.

The seller prefers cash buyers much better due to the extended loan application recently.

You could still use the remaining of the Heloc to buy investment properties and take a distribution of the rents (if you want of course tax implications might be applicable) to pay down the Heloc and repeat process over and over. 

Recheck their lease application.

You can advertise it as "COMING SOON" or something to that affect but you shouldn't be showing it, accepting applications, or otherwise acting as if you own it until you actually own it.

Make sure when you reference check you are not just speaking to the applicant's friend or mother.