I am not too sure about old microwaves. Do you all remember that gizmo that detects microwave leaks around the door? I have not seen any advertised lately. Perhaps the doors loosen up over time and the microwaves leak out.
Anyway we usually get a Panasonic(inverter) from Costco and they usually last about three or four years. A free standing one. Make sure they have an easy to open door when you push the button. $170 can or about $140 us.