Fort Wayne, IN
I can't comment specifically on that belt for your application. But I've used teh Gatorback belts in the past and have been very happy with them.
Judging from pictures of the engine, it should be under an hour for both if they really drag their feet. The plugs look readily accessible without having do work around other components much, and the wires are even more accessible.
How many hours of labor would a shop likely charge to do spark plugs? What about spark plug wires?