You can add a note to your reservation which might be read by the employees in the station... may. You probably can phonecall the station, but that won't change much in most cases - you are still booking a class and not a special car. (i.e. if you rent a M3 etc. thats different)

LDAR is currently mixed by some 525d, 530d and a very few 535d on the BMW side. Audi are currently A6 3.0 TDI 204PS, some 245PS, some of them quattro, some not. Merc just 4 or 5 E350 CDI in whole germany...

SSMR are mostly Audi TT TFSI or Peugeot RCZ, some VW Scirocco.