I often use the Zeiss 25/2; it exhibits little distortion at infinite, but much more at close range (most lenses do that).
The Sigma 24/1.4 A is an excellent lens, and has even less distortion it seems. Since it is relatively bulky, it is not my favourite when paired with the Df, but it is certainly worth considering. MF is easy, but I doubt the FM3a would be able to control the aperture (there is no ring).
Pricewise, a used 25/2 is about the same as a new 24/1.4, but it is difficult to call a Zeiss lens "used", given their build quality.
If you are picky about distortion at a certain range, I can do a quick test (brickwall-style).
I did not mention the 24/3.5 PC, which has a different use case. It is not distortion-free either, but pretty good. Probably not compatible with your camera (the E version has an electronic shutter), but you may find the non-E version used and that one is fully manual.