K mount
- Focal length from 28mm to 50mm
- Interesting rendering, or fast, or something, I don't know
- Modern and vintage are both fine
- Don't want a zoom lens
- $500 or less
What say you?
No need to spend that much! I would start with the ubiquitous SMC Pentax-M 50mm/f1.7. You can buy them all day for $30 -- not because they are bad but because Pentax made a gajillion of them. I have so many I suspect them of multiplying on their own.
For a 28, I've been using a Vivitar 28/2.8 since the early 1990s (one of the first lenses I bought). Stops down to f/22 so you don't even have to bother to focus. Another $30-50ish lens and I think it'll work just fine for street photography. Quality isn't perfect but neither is street photography.
You might want to consider something long-ish. As a kid I bought a Vivitar 70-210 that I almost never used. Someone gave me an SMC Pentax-M 80-210 and I really love it -- seems pretty good for an old Pentax zoom, and it's nice and compact for its focal length. I think you can find them for around $25-$50.
I also just bought an SMC-Pentax M 135/3.5 as part of a $20 lens bundle. Only tried it once and haven't developed the pics -- it's not a FL I used a lot but I am intrigued. I got a killer deal on mine but they still sell cheap, $25-$40.
My next purchase will probably be a genuine Pentax 28, they go for around $60-$75 I think.
If you haven't figured it out, you are VERY luck to be in the Pentax family. They make excellent quality lenses at very reasonable prices. You're unlikely to spend your $500 budget.
For that matter you might want to pick up a backup body. Pentax KX is a more fully-featured version of the K1000 that sells for less $$ (blame K1000 nostalgia). Ricoh and Sears cameras are also great K-mount backup bodies. I recommend the Ricoh XR-1/Sears K1000 (mechanical), XR-2/Sears KS Auto (electronic w/ auto mode), KR-10/Sears KSX (lower-featured XR-2), KR-10 Super/KSX Super (updated KR-10, worse meter display and hair-trigger shutter). I own all of these (well, about to own an XR-1) and they are great cameras -- the KS Auto/XR-2 is actually one of my favorites and I'm looking for a second.
As you can guess I'm a hard-core Pentaxian. I have other 35mm SLRs I love (Nikon FG and N8008, Minolta XG-E, 400si, Maxxum 5) and others I don't love so much (Nikon FE, Pentax MX) and the Pentaxes (and Ricohs) take pictures as good as, if not better than, any of 'em.