After almost 5 years in the game I have a tenant that has not paid, nor will give me an estimate on when they will have funds.

As a retirement plan, the Solo 401(k) is vastly superior, with significantly higher contribution capacity, the ability to make Roth contributions of up to $18K per year with no income limitations, etc.Depending on your ability to contribute, and therefore how quickly you can amass some working capital, you may choose to start with a fully self-directed Solo 401(k) or you may choose to establish a low-cost brokerage Solo 401(k) that would be limited to investing in stocks, funds, etc. for the near term.  

It is the equivalent of raising fund to open a business, purchasing inventory, etc, incurred with a storefront business.

Should we have a joint bank account separate from our own strictly for the REI business to keep track of funds and cash flow recycling and etc?

How would the investment be FUNDED?

However, finding & funding them is the tricky part.

Currently he only needs funding to help cover the rehab completion.Here's a summary of the dealARV: $315k (I've verified that this is in the ballpark +/- 10k)Purchase Price: $130kRepairs to date: $32kAdd'l rehab estimate: $25k << my pieceProposed terms from my colleague:I am offering 13% annual on a 12 month note with a guarantee of minimum 10% return.

You'll need to start thinking about how to pay those sources back in a short time frame since I'm guessing you are a buy and hold investor and your funding sources wouldn't want you to borrow their money for 30 years.

We are not tapping into any equity though and will fund from our personal 3.5% for the next house using an FHA loan.

Generally speaking you cannot use exchange proceeds for rehab (unless you want to pay tax on that portion of funds).