I personally don't use the app and just open BP using my web browser on my phone or Tablet...from what I've seen BP's website is very mobile responsive...Besides having an icon on your home screen & receiving notifications what benefit does the BP app have that can't be accessed from the web version?

Have others been able to secure a HELOC from their portfolio.

All 3 of us have great jobs and great credit so securing a mortgage shouldn’t be a problem.

from the documents a few BP folks sent to me a few years ago. it looked like they were investing in multi beneficiary mortgages IE multiple investors in a fractionalized mortgage.. which can be argued is a security .. in CA they are quite common but you have a specific disclosure document that must be filled out by a real estate broker .. it lets you know who the co investors are.. which is really the rub here.. none of these folks know each other.. and of course this is always the rub in a pooled syndicated investment that goes south.. 

Other than submetering, I'd recommend you ask to see the leases and whether each person has a security deposit that matches their present rent.

Because this deposit typically transfers over to the new tenant's security deposit, should I be charging an amount equal to the security deposit or only a fraction of that amount?

Private owners are more generous with giving back security deposits, are less likely to raise rent, and free furniture is free furniture!

I would talk to a good securities attorney like @Jillian Sidoti to see what issue you may need to deal with. 

Whereas you would ordinarily indicate earnest money, down payment, financing and total purchase price... in an owner finance deal you would spell out the exact financing terms such as:Seller agrees to finance $x of the purchase price at y% interest over z years, secured by a first lien deed of trust.

You will likely even have to get a secured credit card to kick start your credit.