Well they might want to "sidegrade" you into LSAR (4 series coupe) as they consider that a "special" car (and the $ is slightly more expensive in most cases). If they offer you LWAR (same as LDAR just station wagon/ estate /...) that is not a problem either.
You are at an airport so they should have enough cars around. I would chose the 540d if they would give me all options (not the 530i as a gasoline) ...
a) Add remarks to the booking
b) write them an email explaining what you want and why
[email protected] (that should be the one from the Airport directly)
c) if you have not booked (or a change is still possible and not cost prohibitive) use a login for i.e. a Hilton Diamond "9815818" and "diamond"as this sometimes saves you $ and in general states "upgrade if available" and why not ....
[Sammelthread] Contracts / Rabattcodes für SIXT - SIXT - mietwagen-talk.de - Das Mietwagen Forum
d) see if the status match to Sixt Platinum is still possible
https://onemileatatime.boardingarea....-status-match/