The wet end of a pump can well out last the motor so replacing just the motor should fix your problem based upon the symptoms you described. You can buy a new pump if want to but I was suggesting that you don't have to if you don't want to. 12 years is a long time.

If you are replacing the entire pump and motor, then the HP and SF really don't matter much. Either will work just fine. HP and SF only matter if you are replacing just the motor and using the same wet end.

As for two speed, I have a two speed pump with solar on a two story roof. I run solar on high speed and use low speed when solar is not required or during the winter. That alone saves quite a bit on energy costs so don't dismiss a two speed so quickly.