[monochrome]

K135/2.5 for sharp portraits. K135/3.5 for smaller size (sleeper value at market prices). SMC (K) coatings are slightly warmer than the M coatings. Use the hoods. IME green button metering works well with K lenses. Constant lighting? Lock the camera.

[Medex]

lok at this, maybe
https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/233415-soli...m-f-2-3-a.html

[Scintilla]

Only 135mm lens I have is the Carl Zeiss Jena Sonnar 135mm f/3.5 (zebra), but it's pretty nice (and takes the same 49mm filters as so many Pentax lenses). Some samples:
Bird closeups in the yard (uncropped!) -- Penn Station -- Random unprocessed shots around New York City -- First three of these were with the Sonnar and strike me as having a rather "vintage" look -- Dogwood in bloom

[UncleVanya]

perhaps I can contribute by saying what not to go for (this is a relative statement so don't get too upset with my thoughts)

Suggest avoiding the Takumar Bayonet 135mm f2.5 and, to a lesser extent, super takumar 135mm lenses (these show reduce contrast due to poorer lens coatings).

But, whatever you do, make sure you use a hood on the older glass as flare and contrast loss can feature. Once you use the glass with finesse you will get big rewards.

I'm going to suggest that the Takumar Bayonet might not be a bad idea if you put a hood on it and you are careful. My thinking is that this lenses reduced contrast is subtle and might enhance the vintage look indicated in the original post. In addition, the lens is cheap, readily available and k-mount.

I'm under no delusions about this lens, it is not the cream of the crop, but it may fit the op's needs.

[edo]

+1 for the M135 f3.5, lovely lens, compact and light. Very good IQ to boot!

[willdmo]

There are tons of 135 mm lenses out there.

for something unique is often the more seldomn ones.

CZJ Sonnar 135 mm has unique rendering and good quality too,

There is a russian sonner version too, the Jupiter 37 (135mm F3.5). It is the same size as the slower Takumar lens, but has a more classical rendering (its a sonnar type)
Due to its many blades the bokeh is always nice too. Just beware, there are 3 versions. the Jupiter 37 is uncoated, the Jupiter 37A is coated, the Jupiter 37AM is multicoated.

Of course there are good ones eswell from other brands., but lesser special.
to hunt down a good 135mm lens is not hard.
in PK the Porst Auto Tele 135mm F2.8 is fine, aswell as its cosina siblings

in M42 you even can get lucky with a Revuenon, Prinz, or to be more sure, with the larger Brands as Mamiya, Yashica, Pentax, Ricoh, Zeiss, Meyer

[UncleVanya]

There is a nib old stock k135 f2.5 one listed on eBay...

[johnyates]

I think 135mm is a bit too long for portraiture--unless you have a huge studio, or will be working exclusively outdoors. Back in the day it was 85mm that was considered the ideal focal length.

[mattb123]

I have a Leitz (Leica) Elmarit-R 135/2.8 that I adapted to K mount with a Leitax type of mount (eBay knockoff) which replaces the original mount on the lens.
I have only used it a few times and on crop sensors but I'm looking forward to seeing how it does on a K-1. I love the colors it produces and it's built like a tank. They aren't too expensive either.

[Big Dawg]

I like the CZJ 135 f3.5. Not too big and quite sharp.