Here's an example of a mortgage and Cash Flow analysis results.  20% Down payment =  $9,600, Loan amount = $38,400,  4% APR/30 years,  $183 monthly payment

I would have all my docs. run by the attorney, plus have an escrow company for collecting rent and paying the existing mortgage (be sure r.e. taxes are included with that payment also) so it doesn't fall into foreclosure.

But how do I go about paying and tracking the monthly payments to the HELOC.

I've found no fewer than 9 multi family dwellings that are ripe for the plucking but I'm so broke I can't afford a free phone call. by my estimates, these properties combined would net me $8,600 a month in revenue depending on mortgage payments. it is painfully frustrating to see these numbers and not have the knowledge to obtain the capitol to put them in my portfolio.

inherited" tenants like yours are probably waiting to hear from you, so send out contact emergency info and have them respond and include with next rent payment a updated contact sheet,, I'd make an appointment to tour inside of each unit with condition sheet,, the condition from your date of ownership is now what you use for condition move out.. so once that's completed you now know which unit would be tops on your list to rehab and offer an early out to.. if that's a consideration.. you can of course let all tenants know the rent will increase to $$$ when their lease is up and you are accepting 30 or 60 day vacate notices early, they'll be prepared. 

like how much down payment would be added to calculate a positive income for a rental property?

Depending on the lender, you will need to have 6 to 12 months of Capital Reserves, to cover Mortgage Payments.

I've put the remaining $17k in savings and plan to use it as a down payment for a second house.  

I don't even have a down payment!

Also, is it possible to use HELOCs toward down payments and still take out a mortgage on future properties?