What do I replace along with the pump motor?

I'm pretty sure my pump motor is going bad - instead of the hum I'm used to, it's now making a loud buzzing noise. It's still pumping water, but it's getting louder and louder. Is it a good idea to just replace the motor instead of the entire pump? I was thinking about buying a replacement motor to have on hand in case mine goes kaput, or when the noise gets bothersome enough.

If I do replace the motor, what other parts should I replace?

Here is a web page showing the entire pump:

http://www.tropixpools.com/SearchResults.asp?Cat=4031

Do I just need to replace Part No. 1 (the motor), or do I need to replace other parts (all numbered and described below the schematic)?

If possible, can someone who has done this indicate which part numbers I need to replace and why? Is tropixpools a good place to order from or are there other recommendations? They state that they sell genuine OEM motors, not aftermarket motors that fit. Is that important? Does anyone know of a better deal?

Thanks!