My other tenants are fine, but so far I've only had 3 sets of tenants and 1 has gone horribly wrong.

I don't hold that same sentiment but just be aware that if you work with the right person, you can truly have a good experience and not overpay; but if you work with the wrong person, it can be a horrible experience and you can even lose your duplex deal in the process.

I'm comfortable with the glass-doored wall boxes used in hallways for mounting fire extinguishers in common areas; but I'm not sure how to put these within units without looking horrible.

They said that non local owner/landlords were HORRIBLE to deal with. 

However, to buy a house with this problem is really asking for trouble because once you become the owner, you become responsible for anything that happens to that house that the previous owners did not and there are a lot of horrible stories out there about this subject.

Now, perhaps they were into the property at a very high price and had some horrible loan.

After the guaranteed rent period you're on your own.Some of these are not horrible deals.

I was told not to do the deal by another much more experienced investor who just got out of a horrible mobile home deal.

Horrible.

Eventually, you'll have to have a package you send to the bank that includes the contract, the hardship letter (letter written by the owners that describes their horrible situation and why they aren't going to be able to make any more payments), their bank statments, some justification for the price you're offering, and probably other documents.