You need long term debt to make the property cash flow. a HELOC is more like a credit card, it's a revolving debt.

The buyer made it clear she wanted to refinance it in a year once she improved some credit and could acquire an FHA, which I was fine with.

I was a chronically late rent payer way back when, simply because I was poorly organized. 

And finally, make certain you hire a good inspector - you can try to get credits from the seller (based on repairs the inspector finds) plus it's essential to know what you're getting yourself into, property and repair-wise.

Prices are high, expenses are high, and rents are modest so the strategies that can get you enough to live on, to give some examples, might be student housing, higher risk cities with low house prices and poor economic fundamentals, or starting with buying a multi-million dollar portfolio requiring hundreds of thousands or a million+ of down payment. 

How low of a credit score should I accept?

I may never see a dime but at least it sits on their credit and possibly stops them from damaging someone else in the future or keeps them from qualifying for a car loan, home loan, etc.I hope you've learned the following:1.

There is a lot of really poor housing stock out there so you have to keep to your principles and be patient.

Pull out line of credits on them? 

Fantastic to see how well you have been able to do over there, a credit to you.