Use the 20K as a down payment on the next house hack and don't touch your current properties equity.

When revenue is low in the first couple of year, so is the interest payment.  

Based on the CU's loan calculator, we can finance that over 60 months for a $320 payment. 

Not only would it decrease your monthly mortgage but you'd be able to get into it with a very little down payment since you'd be living in one side of it. 

Try to put a bigger down payment in than you might need to give yourself some breathing room.2.

Why move and take on a bigger payment when you’re already locked into such a good deal?

Instead of a traditional down payment, I would give them some money (maybe just to bring the loan back to even).The upside if taken over the loan at the current balance, with some rehab and hopefully getting the front lot back would be a nice ROI. 

The only slight red flag was that they only accepted payment via Venmo or wire transfer, but that's how I accepted payment when I was selling lead bundles, so I still thought it was worth a shot.

On the lease, he'll pay me $100/m over my HELOC payment and all other expenses, including the refi cost, inspections, title work, etc... 

Set up a Joint account with the seller so you can make deposits to continue the automatic payments to the lender until you refinance the property.Thanks,Dustin