Regarding older high quality lenses, at a reasonable price, then the IQ from the Takumars are top-notch and these are legendary lenses. You need an adapter, but these still go for easy access prices.
Else start out with maybe M 50/2, or K 50/1.7. Or K or M 35/2. I was chocked at the IQ of the fast 50 in K-series that I got. And people commended how the pictures were a lot more impressive, than from a Tamron auto focus zoom that I had.
I think what people misunderstand, is that a lens like A 20/2.8 was very rare in the film days. And commanding a price of 1.000 $. Not many bought that wide. But with DSLR, then these wider lenses are becoming increasingly popular, and even harder to get.
The Nikon 28/1.4 is over 3.000 $ on the used market. More than when it was new !
As Canon changed mount, then it is not possible to use a lot of the older lenses.
Check out the work on Pentax Photograhy Gallery, just with the lenses you have. And you’ll see that already now, you can take excellent photos :
PENTAX Photo Gallery
I think it is a fair question that you ask, but highest quality lenses will always be popular. Check out the very old Leica lenses which can be used with Canon !
A lot of fine answers in the thread already. And also a good reply by RiceHigh.