That's a great deal on the shifters with cables(!), but my experience with GripShifters of every quality level is that they are relatively trouble-free, usually not the cause of a shifting problem.

Now if a previous owner tinkers with them, all bets are off. Getting the cable out of it's groove inside, or even (especially) using the wrong lube, renders them useless.

Fortunately, most people can't even figure out how to get into them, so unless the shifter has taken a blow hard enough to break the externals, I generally expect them to work fine.

Especially with respect to aged bikes such as this one, GripShifters do not gum up inside because their special lube is Sil-Tef based, is inert so does not evaporate, nor oxidize, nor permeate plastics, so does not thicken and become sticky with age!
Whereas any Shimano RapidFire type shifter has usually become unusable because of pawl pivot gumming.
Not that a GripShifter cannot have it's sliding cable ramp become contaminated with silt from extensive use, outdoor storage or by blasting with water, but it is a very simple device, comparatively. The very early GripShifters did use plastic indexing "leaf" springs, which lose much of their tension over time, but which continue to work for a very long time.