After the account is set up and funded account owner can use it to invest into virtually anything, including private lending, rentals, etc. 

I then calculate my All-in cost basis (Purchase price, Rehab costs, Holding and Closing costs) using the LTV ratio the lender provides.If it still looks good after the analysis I will visit the property to get an idea of the condition (taking pictures).  

So you want to get pre qualed before you start.. you want to make sure you have Capacity as @Bryan Otteson states you may have to pay the short term loan off in cash if you can't refi or you may need to add a significant amount of cash to the transaction to meet a much lower LTV that a lender may be looking for if appraisal comes in light or terms change..

What is the property type, desired LTV, and DSCR?

Im looking to buy my 3rd property with some vangaurd index funds i have saved up.

I am counting on my rollover IRA to fund the downpayment, and now it's game time to get those funds available as cash in hand. 

I recommend providing a preferred return to the funding partner.

Dont mix personal and business funds. 

We also will have around 7 more SFR's within the next 8-10 years that will not require any out of pocket funds from us aside from legal fees to transfer ownership.I would like to begin focusing on flipping after summer of 2018 to build some larger cash reserves for purchasing more LTR's.

As for investing it into a ETF or mutual fund, it may go up, or it may go down so if you're planning on this loan being the key to start your real estate investing have a backup plan in case the worst happens.