If you want to know what the Bosch part code is, it should be on a sticker on the ECU. That's towards the back of the engine bay on the right hand side. Otherwise go to sparkplugs.co.uk (or whatever it's called) and enter your car details. If memory serves the code is BKUR5ET.
Smart ForFour AMG Black Series Night Edition Premium Plus 125 Powered by Brabus
Try calling the dealer, as parts counter's can be fairly helpful and often prices can surprise you. At least then you know what you need to get prices from elsehere.
dunno, but i got some Bosch Spark Plugs for my 107 last year and it worked out the same price at the dealer than ECP (roughly). May be worth checking out.