There are vendors on eBay that specialize in this camera and have literally 100s of cameras in stock.
Early models are all-metal, while later ones incorporate some plastic, but are consequently lighter. At this age, I wouldn't worry about the reliability difference of the two, and simply plan to replace the body if it fails.
The Pentax K-mount has a lot of cheap, good glass. The 50/1.8 is standard and fabulous, the 28/2.8 and 135/4 are also amazing and can be had for under $50.
The camera is entirely manual, with no autofocus or autoexposure available. Optionally, you can put a button cell battery in it to power a simple exposure meter, visible as a needle indicator in the viewfinder.
I've been shooting these for 40 years, so I'm a bit biased, but I do recommend them just for their ubiquity and cheap replaceability.