Sixt would only guarantee a car class but you can always add a note to the reservation while booking and even ask them to call you to negotiate options and potential upgrades.
http://www.mietwagen-talk.de/index.p...onItem&id=1110 seems to be outdated but their forum/site is well recommended, esp. for Sixt in Europe/Germany.
FDAR has lots of 3 series BMWs in my experience.
In your case best would really be to call them - or try to use a Sixt implant station that is colocated inside a BMW dealership
