I wouldn't lean to heavily on the % rules they are just guidelines. If you have an inspection or walk the property with a contractor or seasoned investor they can guide you in terms of what to expect for maintenance and capex. The biggest ones being a new roof/windows/HVAC, etc. If it's an older building you will have a solid year or two of maintenance issues making themselves apparent (plumbing, electric and handyman type repairs). Just have ample reserves while you get a handle on things and you will be good. If you are buying a large building make sure you get people with the right expertise to look at them. If the building is brick bring a good mason who can point out possible pointing issues, pushing walls, failed lintels, etc.
If you have a flat roof or even a normal roof have a good roofer check it out. With enough expert opinions you can feel really confident about what costs may appear in the first year.
For rent estimates/desireability just contact a property manager in the area these building are located to help you determine those values.