Which 50mm f1.7 - a "K", "M", "A", "F" or "FA" version? Also which 50mm f2.0 . . .
One of the things not commonly assessed is the condition of the used equipment - is it in it's intended condition?
Here are some of my Pentax K mount 50mm lenses . . .
Here are the resolution test results for these -> Pentax 50mm res test
The results are artificially limited to 14.6MP but subsequent tests show they can resolve far more.
Of course this test doesn't show DOF or quality of bokeh - a consideration for faster lenses. All Pentax K mount lenses uses their pioneering multicoated process and works very well to control flare. Likely, for all photographic purposes, the f1.7 and f2.0 will perform similarly.
BTW, if you have the Pentax M42 to K mount adapter, then that gives you access to a world of excellent M42 lenses which are plentiful and usually priced right. Of course even older and condition is more suspect.