Why not try your local Bosch/Lucas Auto electrical specialist for a refurb starter motor. They may be able to recondition your old one on the spot instead. they will do all the oily work also if you dont want to get your hands dirty! this of course reveals that a) I dont know who makes A class starters
and b) that maybe you have to dismantle the car to get at it??? Try also http://baby-benz.com/portal/
A quick look at Euro car parts gives a Bosch starter motor £169+vat exchange + refundable surcharge of £75 on assessment of your old one. this is normal for refurb alternators-starters etc
Also an an interesting online manual on the new A-class at http://www4.mercedes-benz.com/e/cars/a-class/betriebsanleitung/vertiefen/default.htm
doesnt help your problem but maybe the future?