The sash is the main component of every window frame. This will be the only thing to be replaced if the inspection found no water damage on the frame itself and the sash opens with no hassle. In this case, replacing just the sash will only need installing new jamb liners and making sure the design and color syncs with that of the frame itself.
A sash-and-frame replacement is feasible if signs of moisture damage were found on the windowsill and air leaks were discovered on the sash and frame itself. The main window unit is inserted into the jambs and backed up against the stops. There’s a danger, however, of reduced daylight openings if you pursue this option. Going full-throttle on the window replacement is better if the window is totally compromised, but this will involve stripping the old trim and casings.
You have no choice but to replace the windows if they are truly too far gone. It will save you much in utility expenses in the long run.