Here in the state of New South Wales in Australia we have quite a large range of plates to choose from. You can also check the availability, see what they will look like and order online at:
http://www.rta.nsw.gov.au/nd/nd4/ndconfig.nd/OLPS/pgSpecialPlates
If you have time, try it out and see what good ones are still available. Plates to suit most MB models sold here have already gone.
One requirement is that your selection does not exceed six characters. Certain configurations (eg. all letters) attract an annual fee whereas others (eg. ABC-123 or AB-123 or AB-1234 or ABC-12D) only attract a one time fee when ordering.
I was fortunate to get SLC-350 for my 350SLC twenty years ago when I purchased it. You could not do it online then and had to apply and pay your money and hope to get what you asked for. There are many 450SLCs getting around with plates like SLC-014 who probably tried for SLC-450 and missed out. Had I got 350-SLC I would be paying an annual fee which I don't with the configuration I have.
Recently when I purchased my 190E, I tried several like MB-190, MBI-90E and ONE-90E but they were all taken. Instead I opted for my initials and the year we were married to match the plates on our 300TE which are my wife's initials with the year we were married.

__________________
107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.