Well, there is nothing "new" available in the way of Pentax film cameras as they ceased making film bodies years ago.
That said, it shouldn't be too hard getting a really nice Pentax film body for well under the price you have indicated. Depending on how old a body you get, you should consider adding about $85 (US) to have the camera serviced (i.e., a cleaning, lubrication, and adjustment -- or "CLA" -- which would also include the replacement of of the foam light seals and mirror bumper).
I have a couple of MX bodies, a PZ-1p, and an MZ-S. They kind of represent the best bodies of the 1970s, 1990s, and 2000s, respectively.
Of those, I think the MZ-S might feel the most like the rangefinder you have been using. It is a wonderful camera but rather slow for film transport (only about 2 frames per second). It is ergonomically very nice and quite comfortable to hold and use. It prints basic exposure information on the film, which is kind of nice. One of the good things about the MZ-S is that, being the most recently produced model, you can almost certainly skip any need for a CLA. The downside is it has a pretty hard to find cable release (should you decide you ever wish to do time exposures), and the CR2 batteries are a little pricey and can be difficult to get (and it uses these up pretty quickly). Also, because it has only automatic film transport, it can be a little less stealthy (and a good deal louder) than an older SLR.
I would agree with Aristophanes on the LX except that I personally think they are overpriced (especially for one in good condition), difficult to find, and can be expensive to keep in good repair.
The MX would be a very good fit, I would think. Of all my Pentax bodies, its feel and handling seem most similar to my Leica rangefinder. It is, as you've indicated, "something basic" to say the least. If you find one in good condition (this forum's Marketplace is definitely a very good source), and if you then send it to someone like Eric Hendrickson for a CLA, you will end up with a really terrific camera that will last you at least 20 years.
Either the MZ-S or the MX can use any K mount lens with an aperture ring (i.e., the original K mounts, the M series, the A series, and most FA series). They can also use any M42 "screw mount" lenses (e.g., Takumar lenses in the Pentax world).
You mention you want a 50mm lens, which is a pretty short focal length for portraits, but if that is what you want you can take a look at any of the really great "fast 50s" --
SMC Pentax-M 50mm f/1.4
SMC Pentax-M 50mm f/1.7
SMC Pentax-A 50mm f/1.4
SMC Pentax-F 50mm f/1.4
And of course, if you really want to get a good feel for how other people have experienced with these (and other) cameras and lenses, be sure to check out the camera reviews and the lens reviews on this forum:
Pentax Digital Cameras and Film Cameras - Pentax Camera Reviews and Specifications Pentax Lenses - Pentax Lens Reviews & Lens Database
Have fun!