I'm not an expert, nor have I worked on an 04, but it's probably not that bad to work on and would be worth the investment for a fun project.
- Evaluate the filters
- Evaluate whether the brushbar needs to be replaced
+ Are the bristles worn completely?
+ Do the bearings spin freely?
- If not, does degreasing, cleaning and greasing fix the issue? If no, then you can find a set cheap on Amazon/Ebay.
- Check hoses
- Add a light coat of silicone on the seals
- Change belts
That's just a few things to take care of. You'll also want to pull every plastic piece off an clean it. Dirt and stink get everywhere...
The clutch is build very solidly. The belts will snap long before it fails (unless it's on its 10th rebuild...). Be sure to add some grease to the center spindle when you swap the belts.
As for motors, I would personally wait to find another dead vac and pull the motor. However, if you want to do it proper you may want to invest in a new one.