Hi VBlaster,
Good to see a fellow E200K owner - Congratulations on the new car, Im sure you will be very impressed with it as time goes by. A few things you might want to note or check when driving the car - Wiper noise during rainfall (quite significant), Check the fit of the leather trim on the front seatback (The leather on mine was not connected properly, but this was an easy fix) as well as a fairly hard to modulate brake pedal. This is due to the directness of the cars electronic drive-by-wire braking and you will get used to this after a while!

In terms of the Avantgarde-Spec rim upgrade, the cost from the dealer seems quite high for a fairly basic-spec 17" Rim set.
However, if compared to the 18" AMG 5-spokes (retailing for more then $4900 for the set of four) as well as the fact that the Avantgarde spec costs 9000$ when applied fully to the car, it seems like a decent buy. Its really up to you in the end, but the only real benifit of the upgrade will be an enhancement to the cars looks as the tire width remains at 225mm wide and a slightly firmer ride considering the lower-profile tires used.
If you are willing to fork out the cash, go for it. Personally i have stuck to the stock 16" rims, but this 17" set seems attractive as even the rims on the off-the showroom E-class are 16" items with the same Avantgarde 5-spoke design.