I signed a contract with a GC  dated 09/19/2019 to start work on a project and deadline to be completed by 10/23/2019. he missed this deadline, asked for an extension until end of the month, gave it to him.

The repayment terms for a 401k participant loan are equal monthly/quarterly payments of principal and interest (typically prime plus 1%) over a 5 year term (longer if used to acquire your principal residence).Please note that if you take a full $50,000 and then pay back the loan, you can't take another $50,000 until 12 months after the first loan was fully paid back.Per the loan offset rules that went into effect with the 2018 Tax and Job Act: if you leave your job and the loan is current at the time you leave your job but then the loan goes into default because you left your job, you will have until your tax return deadline (including any timely filed extension) to make the loan current by depositing the outstanding balance into an IRA (and thereby avoid the taxes and penalties that would otherwise apply).

Most folks will recommend you do background checks on the sponsors and only invest with sponsors who have an extensive track record.

From my research I have personally done the llc is just an extension of my personal income.

In that case have your realtor submit an amendment for an extension period. 

All new builds in this area are $300k+ and there extensive plans to build highways to this town.

I would hope the work wouldn't be that extensive, but you never know.

(or maybe have some great sources for finding undervalued deals) My girlfriend and I have spoken extensively about doing this, and we are both on the same page.For this example I will use a property that is currently available and listed for $815,000.We would be doing an FHA loan.

How do you get to know a market like this without extensive research that can take months by literally finding out what all the neighborhoods are, exploring all of them, talking to people in all the neighborhoods, etc.?

Ex) Don't give rent due date extensions for a tenant who waves to you everyday vs never giving one to a tenant to repeatedly flips you off.