Coal Ash plant and landfill that closed in 2018
Depends on terrain slope and all. Basically, if any surface or groundwater on your property may be affected.
5 miles from an old city dump
The same gotcha, plus potential odor risks, although if it's rehabilitated (i.e., buried), that shouldn't be a concern.
Also don't have much life experience with this, but thinking that potentially some other company could come in and buy that property and start mining again?
Not sure there's a significant risk of people mining a landfill... you didn't mention having any mines nearby. That said, new mining operations are a risk anyplace rural where there are commercially viable deposits of coal, oil, etc. And the definition of "commercially viable" evolves as the technology improves or global prices increase.