A/C Blows Hot Air Error Code B 1419 EC Light not on

2000 E320 4Matic Wagon 113K Started blowing hot air. Long story short I checked the diagnostic codes and looked at the compressor and clutch. There is only 7 listed for the Freon and the clutch on the compressor is seized. The system was recharged a little over 3 years ago so there is a leak somewhere. In reading the forums I see that people also replace the Receiver Drier and the expansion valve. I understand the drier (is it also a good idea to change the attached sensors at that time?) but what does the expansion valve do? Also planning on changing the O-rings that I remove and make sure that the proper compressor oil is used.

As I understand the process I take it down and have the system purged - can they check for leaks at this time with the compressor dead?, do the repairs and then return and have the system recharged. I have reviewed the parts suppliers that are recommended and will follow them. Only question I have on the compressor is that it appears that this vehicle needs the pulley diameter of 4 1116. Not sure what that refers to but there are several listed.

I do have all the readings from the diagnostic check if you need me to list them. Fuses are fine and I did not check the voltage at the compressor (too tight on the 4 Matic) will do it when I take it out. The grand daughter just said that the A/C just wouldn't work so no real time line on the failure.

Just checking that there is nothing that I am missing before I start and if there is anything else I should consider. Thank You for your assistance.