As you know, the speed of a lens has nothing to do with the viewfinder image you see in a rangefinder. Also, everything will appear to be in focus all of the time, so you'll need to rely on experience when it comes to selective focus, depth of field and "bokeh."
The most critical element of a rangefinder focusing system is alignment, and this been an issue with the Cosina Bessa series. Most cameras are fine, but far too many aren't. This is where a Leica excels, but always check the rangefinder of any camera before buying.
The faster the longer the lens, the higher the need for absolute alignment in the viewfinder.
The Bessa cameras are very easy to use. They're based on the SLR bodies that Cosina used to make for bigger camera companies. The lenses are excellent optically and mechanically.
Rangefinder photography isn't for everyone, and it's not the best tool for every job. Hold on to your Pentax camera(s) and lens(es).
Most rangefinder users also use SLRs. Here's a sort of broad statement: Leica users tend to want to shoot everything wide open to maximize the out-of-focus performance of the lens. This works in some but not all situations, but it's very useful in portrait and when photographing subjects at moderately close distances.
I'm a bit more selective about the aperture, and with wide angles, the aperture begins to become less important.