I am really just looking to avoid the worst of the worst areas (anywhere with open air drug markets or active gang warfare / prolific violent crimes).

The regulatory environment is the first thing you should consider when buying a STR here - it is both the worst and best part of being a CO STR investor.

A lot of voucher tenants actually can afford to not live in the worst parts of Baltimore but won't live in the best either. 

Then, run a rental stress test on the new STR assuming a worst-case scenario—like converting it to a long-term rental with a 30% cut in future rents.

You sound like you might jump at the first bad application that comes in so you need to watch out for that as that is the worst new landlord mistake you can make.

To give you an idea of how we approach it, we assume the worst-case scenario right from the start is 9-12 months of no rent, $5,000 in legal fees, and an additional $5,000 in expenses to deal with moving their stuff, putting it in storage, and dealing with health code violations or NOV's from the city during the process. 

Worst case, slap on some primer, throw down a rug, and voilà: a "blank canvas for the next homeowner."

Quote from @V.G Jason: Quote from @Jay Hinrichs: The mindset of cash flow is everything and appreciation is just luck or gambling or whatever these folks say is one of the worst mindsets I see on BP.

However I wouldn't recommend this or wish ownership of these liabilities on my worst enemies. 

Also depends on what you mean by modest income and bad credit....Worst case like someone else mentioned, you could stay in this property and buy the next with a DSCR.