The Sash weights can be accessed repaired replaced etcetera once you have access. Typically this requires removing one of the window stops. Alternatively there are replacement equiopment that can be used. This involves fastening a spring mechannism to the window and the frame.
As for replacing the glass you will still have the problem of a low insulation value if you leave them single pane. You can have insulated glass installed but this can be an expensive process as it requires nodification to the existing Windows.