Broadly speaking, although the fleet changes frequently, here is a good list of what sort of vehicles you can expect from Sixt in Germany on an FDAR/FDMR reservation:

Audi A4 (1.8 TFSI, 2.0 TDI)
BMW 3er (316d, 318d, 320d, 330d, 335d, 316i, 320i, 328i,) - teilweise auch mit X-Drive (Allradantrieb) - Some equipped with X-Drive
BMW 3er GT (318d, 320d, 330d, 335i) - teilweise auch mit X-Drive (Allradantrieb) - Some equipped with X-Drive
MB C-Klasse (180, 200 CDI, 220 CDI)

As for a rate change for early drop-off, it's hard to say without knowing the exact rental details.

Start and end dates and times would be useful to give us an idea.