I'm exploring SD checkbook IRA thanks to the advice on this forum.

Does anyone know where I can find a definitive guide on self-directed IRAs vs Solo 401ks.

Does anyone know how or if it is possible to transfer a Roth IRA to a SDIRA? 

I know it's a risk but what the heck I figure I can afford to risk that much out of my IRA instead letting it disappear when the predicted inevitable bottom out happens.  

Self directed IRAs, for investment property, may offer another route because the loans are non recourse and don't show upon your credit.

So offering an 80/20 1st lien position for 12% interest only payments has been very well received so far.Even if you're not in a city like Denver, there is trillions of dollars out there in stagnant IRA funds collecting no more than 1% interest so I don't think it would be a problem for you to find someone willing to make 6% 8% 10% or 12% on their money, and have it secured by real estate!

As we considered our options to pull together funds for a down payment, we considered closing our Traditional IRAs.

I was looking into details/scenarios on funding an investment property through a self directed IRA.

We accidentally acquired our first rental, and it wasn't until after our first tenant moved in that I did the math on it and realized that we would be earning over 15% on our money; that's when I got the REI bug.This might not work with your wife if she's feeling that the retirement savings/IRA/401k route is better.  

Namely: Diversification of my overall investment portfolio: Despite my IRA goals, an REI opportunity would help diversification.