Here is my question: Should we take out a HELOC (home equity line of credit) or a home equity loan on our existing SFR home (that is paid off) to finance a 10% down payment on the new investment's mortgage? 

I agree, I'd call the power company emergency line. 

I also believe there are lots of videos on YouTube, some selling products, but many just offering guidance on things that they have tried and worked or didn't work.  

I would not do this deal for the following reasons:-Austin has had a tremendous run and while I do not see a huge downturn, I do not see how the run can continue at this level-Huge number of rental units coming on line that will bring down occupancy levels and rents will follow-Your $160 per month HOA fee WILL jump once the complex is sold out. 

Look at the 50% Cash Flow line.  

Also down the line you can get into road block getting more heloc like you mentioned the snowball effect.

Bottom line is the IRS is always skittish of them. 

During their introductory time to the end of October 2018 they offer to be able to borrow 3 times at a special rate of only 3.99% with their EQUITYLOCK product.

For the next property, take care of this one, and down the line you can possibly look to refinance (if the numbers work) so you can have capital for the second deal. 

Part of the appeal of this property is the fact that it's relatively close to me as well as the potential for capital gains 10, 20 years down the line.