I purchased a J-12 attached to Zorki 4 - but too be honest I never shot a roll with that combination (not sure why). I have, however, used the J-12 with my Bessa R and I found that worked well; mostly because I liked using the very clear and bright finder and because I didn't run into any problems with focusing incompatibilities - but then I never went more open than f4 and so depth of field was fine and covered any minor incompatibilities that I might have experienced. I also preferred the Bessa over the Zorki, Fed 1, or my IIIf because it does have internal finders for 35 mm & 90 mm and I used a CV 35/1.7 with the Bessa body.
I've also used a
Russian turret finder but I found that clumsy to use because it seemed to me to be awkward on top of the camera and really the combined size defeated the point of a rangefinder for me - I tended to think - this was especially true of the turret rangefinder on top of Leica II-III's and the Fed Zorki derivatives (although ymmv). I've also used a
Helios (35,85m,135mm) finder which is much smaller than the turret finder and may be worth considering. Not that it's as good a finder as the Russian turret finder (which has correction for parallax, separate views for each focal length and a much clearer view). The Helios has 3 focal lengths all in the 1 view window (if that makes sense) - but in terms of letting you frame a picture it's a snap to use. If a little yellowy amber to look through.
However I find all of these options to be less pleasant than the Bessa's viewfinder: If only for its combined RF/Viewfinder and the fact that I find using an external viewfinder somewhat clunky. Of course if you don't mind focusing and using a seperate viewfinder then this may not be an issue for you. I love using my CV 15mm with its viewfinder - but then I'm not trying to use the rangefinder in that instance.
Of course J-12's are a lot cheaper than Bessa bodies. But really, why fight the urge? Get the J-12, a Bessa, an M3, a Zorki, a Fed, another J-12 on the end of a Kiev, a ...
BTW, what 35mm / 90 mm lenses are you considering?