If a property needs to be rehabbed, what is the best way to calculate an ARV so you can work backwards to see if it is a good investment opportunity? 

BiggerPockets have plenty of info how understand and avoid predators, however, I don't want my excitement of my first deal to get the best of me either.I see the BiggerPockets Calculator but I'm still stuck on:  where to find HML/Private Lenders, going interest rates, average points (1 to 20), making sure to include quiet costs....etc.  

Your relative should be aware that they have 5 years to pay that loan back or it becomes a taxable distribution.

Try using the BiggerPockets Flipping Calculator to run the numbers and see it from their perspective.  

If numbers aren't your thing, you can still use the calculators under the Tools menu.  

Set to be completed in the Summer of 2017.The Following Info is according to the information from the Developer:Purchase InfoInitial Market Price: $700,000Purchase Price: $649,900Initial Cash Invested: $187,475Income AnalysisNet Operating Income Annually: $52,484Cash Flow Annually: 22,847Financial MetricsCap Rate (Purchase Price): 8.1%Cash on Cash Return (year 1): 12.2%Internal Rate of Return (year 10): 19.6%Sale Price (Year 10): $896,059Rental Income is forecast to be $1,400 per unitUnits are 3bed, 2 bath and a garageUnits are 1,600 SqftI feel that the seller is optimistic on some of the numbers so in my calculations I have increased expenses (CapEx budget, management Fee at 8%, Insurance, Utilities) and I have also increased the vacancy rate from 3 to 5.2%. 

Masshousing has a max income limit, if you're buying a multifamily they include the rental income in this calculation which can be a deal breaker if there are two income earners.  

Since Wednesday (two days ago) the use of the rental calculator has actually already helped me review four potential deals that I was able to then, print and hand off to my lender yesterday (Thursday) during a meeting, and he was super impressed with my presentation.

Use the Rental Property Calculator to make sure it is a good deal.  

My agent tells me to think long term. https://www.biggerpockets.com/calculators/shared/545686/5e8f750a-f3c0-43f8-8fce-6316474e20f8