Play the long game and focus on rent and home price appreciation.

A HELOC gives you flexibility since you can draw as needed and only pay interest on the amount you use, which might be useful if you’re not sure exactly how much you’ll need upfront.That said, it’s important to compare the interest rates between a HELOC and your expected returns in the stock market—if you’re making more in your investments than you’d pay in interest, it could be a good play. 

We are under contract on another right now, but it's an appreciation play.

Again several factors at play but just providing information based on your criteria.

Right on cue, we see 0 to 1 at play from @Eric Carr.

But there are other factors at play.

The property is a 4-unit building, in a good location for the aforementioned play.

Properties north of Greenridge St. are a fair play. 

Quote from @Gerald Wallace: I'm playing Devil's advocate and running your numbers through the worst-case scenario.

The smart play would be to build a two story wood frame structure on top of piers or concrete block in most cases. 35 feet is a pretty typical height limit in most neighborhoods, so depending on elevation you SHOULD be able to build two stories on top of the elevated foundation.