MB now warns about brass keys. The brass particles deposit themselves in the tumblers and cause problems.
Lock cylinder problems with the ignition switch and steering column lock in an MB, is a problem that you want to take all precautions to prevent. If one of these locks fails, it can cause you to have to remove the steering column to drill out on many models. It's typically about 6 or 7 hours work.
Don't take a chance on a brass key. It's not worth the risk.
'01 C Class, Six Speed
'84 Euro 240D, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in