How much do they want for it? 15-91's are my favorite Singer. They are gear driven, all metal work horses. Keep them oiled and don't try to sew steel plates or saddle leather and it will out live you.
Most 15-91's will have to be rewired. The wiring is 50 or 60 years old at least! The places to look for dry, brittle wiring is at the lamp and behind the power block. When you remove the 3 prong cord from the machine look at the receptacle on the machine; there is a screw holding it on the machine. If you remove this screw you will have access to the wires coming from the motor and the lamp. Check them carefully.