Ruben, the site you might want to look at is
www.pentaconsix.com.
There is a seller called Cupog on the auction site who does CLA'd P6's and I have bought mine from there. There have been no problems at all, over a period of more than a year. As these machines are definitely not new any more, it probably comes down to the history of the specific camera - just like all our old cameras I suppose.
The P6 shutter and mirror are very light and quiet, compared to a Kiev (one which was not adjusted recently, almost certainly) that I have used. The viewfinders, prisms etc are in reasonable supply and the breech-lock lensmount is the same as Kiev 60.
I have some doubts about quality of the Kiev, Hasselblad-like, copies due to the complexity - but it is still also possible to find them from a source that rebuilds and guarantees them (eg.
Araxfoto) although I have not tried that route, as the P6 works fine for me.
Medium-format knocks 35mm in to touch for quality of results for sure, and these cameras are excellent entry points to the format.
Good luck and keep us informed . . . .