Hello Marco Lucifora,
Before you get too far into this please read the threads here and Photo Dot Net about these conversions going back as far as maybe 2002. trhere was a good bit of drama amid claioms, patents and threatened law suits. Please use the search function and read a while first.
I have never owned one of these cameras, but I have handled a few. It is my opinion (and it is just an opinion) that the Byron cameras are the most thurough and comprehensive re-build out there at a realistic price point. Check the Byron videos on YouTube.
As far as how large the market is: Years ago there were threats of court action over patent infringement. Nothing ever came to court. If there was any real money to be made in this it would have come to a court fight. If there was a real commercial market out there, someone would be making folding rangefinders in 4x5 from scratch in China.
with respect for all, cheers.