$60k in this property could be invested in Bitcoin or Apple stock or a SP500 index fund.  

Bids were given as a margin over some index sometimes, or a fixed fee above the spot price the day of install or scheduling etc...the client had the risk of oil going up or down and then pricing was locked as the work was completed.

Look up the 100 year Inflation-adjusted Case-Schiller index and check out 2006-2014.

Should I go with that loan, contribute the money I am earning now to a ROTH IRA & index funds, but pay interest later on when paying back the down payment loan.

Push money into Other asset classes like stocks or Index Universal/Whole life policies?

Nevertheless, this is what I would recommend:Take $3,700,000 and put it into a vanguard index fund that tracks the whole market or the S&P 500.

Compound interestRegardless of your real estate investments/holdings... regularly save and invest in a  low cost (by definition) index fund.Appreciation for compound interest is wasted on the old!

According to the case shiller housing index, historically, markets in large cities, especially the west coast and NY have peak every 18 years on average.

If you're okay with a little more risk, a bond index ETF with a very low expense ratio might make sense. 

Do you invest it in index funds to make it grow while you save?