My plan is to use FHA, 3.5% dp, seller covers closing costs. 

My point is that in a market where overall demand is unbelievably strong, the opportunity-cost numbers are not that strong for an all-cash, or any for that matter, acquisition relative to past history. 

We can do some of the legwork ourselves to cut cost - we have flipped houses and built two new homes before but we had the misfortune of starting in 2005 and ending very abruptly in 2008. 

This creates two loan payments ($100,000 of equity and $300,000 on the new mortgage).Key NumbersHome Equity Loan Interest Rate: 6%Mortgage Interest Rate: 7%Rental Income: $3,000 per monthExpenses (management, taxes, insurance, maintenance): $800 per monthIncome and ExpensesMonthly Rental Income: $3,000Monthly Expenses: $800Monthly Mortgage Payment: $2,000ExplanationThe investor earns $3,000 in rent each month.They pay $2,000 on the investment property mortgage and $800 on other expenses.This leaves $200 profit each month or $2,400 per year.However, you have to pay $6,000 interest on the equity borrowed.This leaves you with an annual loss of $3,600.This example shows that while the rental property generates positive monthly income, the interest cost of borrowing the initial $100,000 results in an overall annual loss.

There are some scenarios which can change the equation and take priority over the opportunity cost equation demonstrated above.  

You seem “golden” in any court case when the city inspector was out and didn’t notice it, they didn’t notice it, and you twice agreed to let them go at no cost. 

I’ve heard about ‘gap funding’ for fix & flips to cover down/closing/holding costs in addition to the hard money loan for most of the purchase price & rehab, but how realistic is it to secure that type of funding?

New builds are far more desirable, and have less total cost of ownership than many of the stick framed, pre-existing, non conforming older homes. 

It's going to cost about $550k to build and it has already appraised well over $800k.

Be sure to no only account for the cost of the cash for keys but your additional holding costs while you have to wait to get them out.