Much research, tests and reviews of vintage lenses, especially for use on digital DSLR cameras can be found on the comprehensive site of Vintage lenses for video
Lately i am looking and reading more and more of the interesting tips & reviews from the spontaneous Mr.Leica (Matt Osborne, and it is not all about Leicas).
My favourite Youtube channels with lots of reviews of vintage cameras and lenses:
The Art of Photography: Ted Forbes and his reviews with videos about photography.
Zenography: Thoughts and reviews of cameras and lenses both old and new.
Reliable vintage camera shops (no affiliation!):
Petrakla camera’s (Holland, all vintage cameras with CLA!)
Analog Lounge (Germany)
DeWit Cameras (Holland)
Repairing or CLA for a Leica, do not hesitate, the legend lives in Holland: Will van Manen at www.kamera-service.info
And if you are patient, it will be rewarded on catawiki, every week hundreds of vintage camera’s and stuff.
And if you ever need a manual to one of your old camera’s, the most fantastic site belongs to Mike at https://www.butkus.org/chinon/ My compliments for this historic database of all camera’s.