But also, to demonstrate even more: whose NOI are you using?  

How you approach the mentor demonstrates how you will approach real estate investing.

The fact that you have to ask this question demonstrates your PM isn't very skilled.Your PM should have a policy that prevents this from happening.

Further, how do I demonstrate proof of funds on a HELOC (ie, the equivalent of a pre-approval letter for a loan)?

@Jay Hinrichs demonstrated the challenges and shared a fairly narrow fact pattern where specific performance is plausible.

If you are not assertive and demonstrate seriousness, tenants will often exploit the situation and not perceive you as being serious!

Demonstrate to your partner how this could help you live a freer life later on and maybe they'll get on board.

For credit score requirements, Washington state law discourages setting minimum scores unless a "business necessity" is demonstrated.

I would only consider this if they can demonstrate they are a professional company with a long history (operating 5+ years would be my standard), and if they are willing to make it financially beneficial to you (higher rent rate, they handle maintenance, etc.).There are a lot of "businesses" that started recently that have no real experience, meaning they've only operated in a strong market.

I really mean professional as in: proving that a PM has a sizable portfolio, documentation and forms that demonstrate processes that make life for tenants and owners easier, and software that keeps communication easy.