I have come across Advanta IRA, Provident Trust Group, and iPlan.  

I was looking to self direct a Roth IRA, roughly 20k.

Look up ROBS roll over business startups, checkbook IRAs , etc. your age, income, employees, time spent in flip business will effect your options and outcome.

I WAS able to take out a new loan on my SD401K (I had already set up my SD401K with other rollover IRA funds) and COULD have used those loan proceeds to pay off the company 401K loan prior to transferring it over, if that makes sense.

You won't be able to do a HELOC on IRA-owned property.

I'm under the assumption that my other IRAs cannot invest in this project because the GPs are now disqualified to me personally or to my other IRAs. 

From real estate sales, wholesaling, flipping, rentals, lending, self-directed IRA and Solo 401k investing, or tax and legal guidance, you will find that many members here are very knowledgeable in their respective fields.

He's willing to move his current IRA into an SD account so to have checkbook capacity, however, we found that since we are his children that he is unable to lend/co-invest with us(disqualified persons).

A few of my investors have inquired about investing via self-direct IRA, so we will likely make that an option. 

So is that similar to investing with an IRA.