The husband had been in the military & had an accident which caused PTSD.

You may have a hard time being flexible when unexpected hardship or opportunity shows up. 

Add another 10% to 15% for unexpected issues and see where it puts you in regards to the ARV.  

Under $500k is already very difficult (only about 30 listings in the whole city at this price point currently, with 10-20 buyers competing on them) and not getting any easier anytime soon, barring something completely unexpected happening of course.

. $1800 in unexpected repairs + $100 in DIY drywall repair.

"I will pay you back, we just had some unexpected expenses this month."

Let them know, in very clear terms, that the contract is contingent upon your inspections, and if you find unexpected details about the property, you’re going to need to adjust the offer/contract.

That isn't a lot of wiggle room if something unexpected comes up.

If you converted your SFH to a duplex, were there any unexpected problems/surprises on the legal/regulatory side, like being unable to rezone the property or get a rental certificate? 

If I had to choose some must have metrics, I would say DSCR and CoC are perhaps the most important to me because I need to be certain that there is some element of buffer from the risk of losing the property in the event of a market shift or an unexpected issue in the structure of the property that causes vacancy or expenses to rise.