Initial questions to you:
1. Do you frequently use the AC? Or at least once/twice per month just to lubricate the system up. If you do this then the only wearing part is the pulley's bearing as it turns whenever engine is on.

2. What made you think there's a leak? Less cooling now? Low or zero refrigerant pressure? Found oil around compressor? Refrigerant loss could happen thru rubber parts - O rings, hoses - with time. Thats normal.

3. What compressor do you have now? The one originally came with car? York? Diesel-Kiki? Sanden or Zexel? Specific compressor needs specific repair procedures.


To answer your questions:
1. Compressors most commonly leaks around the shaft seal. The shaft seal is located at the spindle deep inside the front assembly of compressor. Specific tools must be used to replace the seal. Watch this video for general idea: https://www.youtube.com/watch?v=sh-KIjAqh-A

2. Yes. Thats what happens to used compressors sent for rebuild.
Watch this video: https://www.youtube.com/watch?v=0fZXqLeIkNc

3. Yes. If its just a simple DIY test then put your thumb on the suction port and turn the clutch assembly (not the pulley) in the direction of engine turning. There should be suction felt and your thumb stuck at the port. That should tell the compressor is still good. For a more complete test it should be done with pressure gauges as in the second video.

4. For noisy compressor see Sanden's list of compressor noise (generally applicable to other makes too): http://www.sanden.com/objects/Compressor_Noise.pdf

Hope these helps,
Amarin.