I’m of the school that believes the first step should be a compression check to make sure the engine’s top end is good. It’s stated that the car was not taken care of too well. Verify the engine is half decent before spending money is my premise.
A precursor to a compression check could be connecting a vacuum gauge. That is quick, cheap, easy, and insightful.
If the engine is dropping compression on a cylinder that can cause the spark plug to fail. That in turn can cause the coil to fail. Replacing plugs and coils in a dropping cylinder can be an exercise in futility. Hopefully that is not the case.