2003 W209 CLK, 2005 R171 SLK in UK, 1995 S124 E280 in Italy
The keys should all be the same. Some if not all the locks have been replaced at some stage. Main dealer keys are cheap unless it has a remote but they will only supply keys for the original locks. Its going to be difficult if the door locks are not the originals. Might be easier to get secondhand locks with keys from a breaker
There are a few C-reg W124s about but I don't think I've seen an estate earlier than D or E
But you never know with 124s!
If the car is a W124, don't assume the doors have different locks. Non-turning locks are a W124 speciality. Remove the door lock barrel & trim and try cleaning it with solvent then oil then solvent, etc. This may bring it back to life
The barrel can be removed - at least on later cars - by removing the bung in the door jamb, loosening the hex bolt and sliding the barrel & trim out of the door
If the ignition barrel uses a different key to the rest of the car then the ignition barrel has been changed. Also common
Another update - I noticed that the drivers tumbler was not moving at all in the barrel, so I put a good supply of WD40 in and tried it again. No joy. For fear of breaking the key, I used a large flat bladed screwdriver (gently!) and it moved! I then put the key in and it moved all the way!
Success was short lived as the lock can now be opened by any object that sits in the slot!
So, I still need to remove it to sort out an opposite problem!!