Talk to a mortgage broker or lender experienced with LLC-owned properties before transferring ownership.

You're going to end up paying a lot more interest on a heloc than a 2nd mortgage, if you intend on holding that debt for more than a couple years. 

@Kris TohovitisI wouldn't recommend using your cash to pay off the mortgage on your primary residence since you can likely get a better return on that money by investing it.What's your buying criteria for the multi-family property?

In Massachusetts, helping your non paying tenants find new housing that they can afford and paying for their moving costs and new deposit can be cheaper than the long eviction process.

If you are doing rentals, especially long term rentals, you are unlikely to be able to live off of those for a while if they have mortgages. 

@Chris LinIf you were to include Equity growth (from appreciation and mortgage pay-down) what sort of returns would you have? 

For full protection, your mortgage company will likely still require you to carry a landlord or dwelling policy.

I wanted to offer him about $50K over what is remaining on the mortgage and go through the 60-90 day process to assume the existing mortgage.

Even at 30% discount, the QBO Plus subscription most Real Estate investors have to have is still $63+Tax.You may look at Xero, I heard they have the Class feature but it is cheaper, it's just not as commonly used as QuickBooks.

Research ways to use your education to help you qualify for a loan (did you know that some lenders allow college credits that translate directly to the line of work the graduate pursues to count towards work experience, accelerating the borrower's opportunity to get approved for a mortgage?).