Pat, there is a good rangefinder site,
www.cameraquest.com I remember the write up on how to load that Leica-pretty tough but worth it.
The Leica uses screw mount lenses-many are still available as is cleaning and service. The focus screens can get hazy over time but can be cleaned. The shutter of the Leica is a thing of beauty-whisper quiet and very precise.
The IIIf RD ST is the most sought after of the series. None of the III's have as much resale value as the M series. That is the camera that popularized 35mm and those knurled nobs and engraved top are amazing. If you've never heard the story before look up The Leica Freedom Train about Leitz role in helping German Jews in WWII. It looks like you have the 50 and 35? The 50mm view finder slides into the shoe. If you haven't been able to ope it yet-the bottom plate comes off and the spool comes out to load it-not easy.
Did you notice how Leitz uses the cm scale instead of mm? I'm not sure when it changed to mm.