I agree LTM Leicas are a good way to start in RF, if you want mechanical reliability and interchangeable lenses.
Here's some good info.
Canon also made some similar cameras in LTM mount, and you may be able to find those for a bit less $. I recently sold a 1953 Canon IVsb for $125 here on the forum that was in good shooting condition and had new curtains and a CLA by Youxin Ye a few years ago. Canon also made other LTM bodies that were a bit larger and slightly more user-friendly IMHO, like the Canon P and 7 models.
However, for shooting experience, the
Leica M cameras are worlds better. A recently-serviced M2, M3 or M4 will go for a decade or more without any problems, if you use it regularly. The combined RF/VF is a joy to use - simple and clean. If you lean towards wider lenses, I'd recommend an M2 or M4 as these have built-in framelines for 35mm lenses. If you usually shoot 50mm or longer, an M3 may be better.
There's also older Contax mechanical rangefinders, but make sure the shutters are in good condition. Repairers are fewer for these than for Leica. Nikon RF's are another option, but probably harder to find one in good condition. I bought a Nikon S2 from KEH a few years ago, only to find the slow speeds were unreliable. KEH sent me 2 replacements, and they both had the same problem. I'm still looking for a good one.
I have a Voigtlander Bessa T as a backup to my M2, and it's been a great camera, but it is electronic-dependent. Prices on the
various Bessa models seem to be going up since Cosina discontinued them a few years ago.
There's a lot to choose from, so Good Luck!