This is my first purchase, where my intent was to install recessed lighting all throughout the house.

But I purchased land in cash and leveraged it to build a new construction stick built home.

refinance our own home with some money down + purchase another flip? 

And what is your price range on the purchase price?

I'm looking for advice on whether I should purchase my father-in-law's 6 unit apartment in Burien, WA (10 minutes south of Seattle) as my first deal, or perhaps help find him a seller that would pay more than I could and is a couple years ahead of me in REI. 

You get a Capitalization Rate (CAP Rate), which equals your annualized return by dividing the Net Operating Income (you had gross in your narrative) by the purchase price.

(You said you could purchase them $20-$15,000 below market value. 

It looks like "Good Egg" is simply an Entity that collates funds, and then invests those funds on your behalf with the actual sponsors/general partners that actually purchase the properties, get the loans, manage the properties and distribute the funds.

Has anyone purchased Real Estate with cryptocurrency?

If the proposed loan is too risky because (1) chance of default is too high and or (2) cost of foreclosing too high or time it takes too long and we would otherwise decline the loan, we might offer to purchase the subject property for a price equal to the loan amount.