C111 is no car you can add into this game.. It was a vehicle primary designed to test new technologies.
A45, this car is very important to Mercedes. They want to attract younger audiences. Adding this car to GT5 will help them alot.
E63, GT5 really lacked a premium E class AMG. We had a A6 and M5, but only standart E55 AMGs to race against.
SLS GT3, maybe we will get more GT3 cars, so we can have full GT3 grids. Also, this is the only Mercedes premium race car. For me it was really underwhelming in GT5 that my favourite brands (Merc, Ferrari) had no GT type race cars.
Btw, maybe we get a RMed C63? Mercedes used the W204 pre facelift (the one we have) in DTM till the end of the 2010 season, then it got replaced by the facelift car and a year later by the coupe.