I really need to get this house under contract and get them some cash, but I need to be armed with info to take on this landlord.

Hello Devon,   first and foremost thank you for your service in the armed forces !  

The fixed-rate is important not an arm.

But in this case, since you're buying from your mom and you can include in that contract and deed explicit permission for a wrap, it's not a big deal.However, anyone who's going to buy a $40,000 house isn't going to be able to afford $700 per month, and if they can afford it, they can likely go to a bank and get a traditional loan.

If your going to landlord in a lower end area with the threat of violence at anytime you may want to arm yourself.My PM's that I work with in pretty much every city,, if they work C and D they are packing for sure. especially if they walk in a vacant home

A writing is not explicitly required.

I want to flip in KC, Omaha or Memphis, these markets are all within an arms reach of where I am now and I'm ready to relocate. 

This creates exposure to Unrelated Business Income Tax (UBIT).There are good things you can do with a self directed IRA or Solo 401k, but it all has to be arms length.  

Are these good ARM loan options for multi-unit investments properties?

So there might be an office that says they don't want part timers but another office with the same company will welcome you with open arms.