The value for an entry level 03/04 C Class is around 16-22.
Though I would NOT suggest the purchase of a 3 series BMW or C Class Benz. Youll find much more in a high end CL-9 or Mark X 250G. Youre paying the same, but the Honda/Toyota is a "top of the line" in its class while the Mercedes is an entry level with less power, interior space, comfort, technology & fun.
Overall, getting a 2006 Mark X 250G in 23 is much much better than a Benz C200 or C230 - you get way more power, comfort, luxury and all. However, if you want to just say "I drive a Benz", than even the best Toyo wont cut it!