I was once interested in buying a used 25/2.8, but it could not focus to infinity and the shop (a Leica, Voigtländer and Zeiss dealer...) confirmed it by testing it on a bench - but they did not offer to adjust it.
The 25/2 is probably the better choice (low astigmatism, low coma...) but field curvature is the other way around, so you've been warned. By the way, my copy was for sale over about 6 months until I finally bought it. You may want to check Photo Ciné House in Brussels, rue du midi. They also got other second hand ZF2 : 35/2, 50/2, 50/1.4, 85/1.4... and the boss is yet another Df hugger
It is not the first used lens I bought there and, unlike several Paris shops, they never try to cheat people by selling stuff with semi-hidden defects. In Paris I experienced :
At the Boutique Nikon, XVIIth district:
- A grossly defect Noct (1300€ but that's no excuse), which I sure did not buy
- A slightly defect 50/1.2 AIS (diaph does not open fully, maybe 1.25 only); they did not tell nor admit the defect, but sharpness was good and price was low, so that was a deal;
Somewhere in the XIIth district:
- A defect Zeiss distagon 15/3.5, never sharp even stopped down; not bought;
Photo Suffren, XVth district:
- The above mentioned 25/2.8 and, once again, I discovered the problem (and did not buy).
I can only recommend, in Paris : Photo cirque and Le Moyen Format. They treat their customers seriously, not just their pals.
In Brussels, of course, PCH rue du midi.