The Jupiter-12 can be awesome, just be aware that you may have some fitting to do. I had a little decentering on all my FSU lenses and I had to do a little work on them, but the J-12 is a fantastic optic, virtually no distortion.
As previously mentioned, the 3.5cm f/2.5 Nikkor is a great lens. I got one off of craigslist in Jersey City, NJ maybe 8 years ago and it was the lens that began my conversion to Nikon RF glass over Leica. I just preferred the look and especially the price. That first Nikon mount 3.5cm lens cost me maybe $50. It was the heavy chrome but that was fine. It made beautiful images.
Phil Forrest