Considered you already have a 135, an 85 sounds the right choice. 85mm is, however, my favourite length for portraits.
I own a good old Takumar 85mm f:1.9 that I occasionally use on a (old just as much) Voigtlander with screw mount, and of which I can't say but good words about. If used wide open, it shows a pleasant natural flou effect which I like very much. If hands movement is an issue to you, this lens' great aperture would let you use very fast shutter times. Depth of field is very shallow, but you understand that the definition on the final print depends as well on magnification of the negative, and you can always close the aperture if you need.