looking at the current fleet at sixt... ldar is more likely to result in a e280cdi
but munic airport also has a lot of xdar... maybe you can persuade them to give you a 730d. (when I was there 2weeks ago they had probably 5x audi a8 3.0tdi)
otherwise I can only recommend the way "ulev" suggested.

btw, the 5series at europcar are either 520d sedane or 530d estate.
but the majority is e220cdi or worse: e200komp.

in the end its luck and difficult to influence
but the e280cdi hat a map with the navigation (which hasnt been the case 6months ago), so that wouldnt be too bad either, even though I admit the 530d is much better