around mid 2004 saw a facelift for the C Class so depending on if the 54 plate model is a facelift or not there could be quite a few differences.
The later facelift models are built a lot better in my opinion - interior is much better built & looks a lot nicer. Exterior changes were a little more subtle - slight changes to the lights, trims & wheels so can look very similar on the outside on a first glance.
I would personally recommend a later facelift model - the 54 plate as said may or may not be a facelift depending on when it was registered compared to when it actually left the factory (it is not too unusual to see some cars sit around before being registered)
Looking at the outside a facelift models headlamps have a smooth clear lens where as the pre facelift has ribs in the lens.
Looking inside the facelift model has a larger "double DIN" style radio where the pre facelift has a narrow single DIN style (unless COMAND is fitted)
Let me know if you need anything else.
Best regards, Paul