Basements (if they have them), roof, attic, crawlspace, exterior, and all mechanicals are where all the unseen costs are not in the kitchen, bathroom, etc.

You asked about buying older properties and the biggest issues I have found is that roofs need to be replaced, HVAC needs servicing/replaced, and sometimes there are foundational issues (not super common). 

I live in Georgia and the hurricane destroyed the roof.

Parts of it are similar to SFR, some aspects translate, but they are different worlds in different universes. 

If you’re house hacking there is an aspect of your lifestyle to consider.

I think you need to weary about several other aspects with an STR. 

As Hurricane Helena went by, both of the roofs got damaged.

If they have a good roof, I use a company that used to send someone out to do a roof inspection (at no cost to me). 

And in some cases, a roof inspection when the HI is not confident or the roof is a specialty roof (metal or tile) 

Unless you are doing a serious amount of volume , both from a cost and performance aspect, it will make more sense to outsource.