Does anyone have any calculations that help them decide how much cash to put into a property factoring in opportunity costs for that cash?     

Those engines last forever, but at that mileage other parts do begin to break so you'd have to manage that, especially depending on the previous owners care.

I don’t need them to help me run the RI business but I do need them to factor it in properly to my overall financial picture and retirement planning.

According to RS Means national pricing list, Alabama is close to the National Average if not a little cheaper.There are also a number of factors that can influence costs, but the pricing assumes a 3 bed, 2 bath 1,500sf average grade rehab. 

It would depend on multiple factors.

This, combined with economic factors such as the California exodus (and other migration patterns) lead me to believe there will be long term growth here. 

So depending on size and other factors you can budget another 5-25k for a rewire and Drywall repair.

Since I have renting experience (which I've read is a qualifying factor to be able to use projected rent income towards DTI) will I be able to use that towards my next multi I plan to do an FHA on?

You have to factor these in to see what kind of actual cash flow potential you might have. 

But it takes forever for the closing to happen.