Originally posted by Cavelle ymmv?
Your mileage may vary - you might make different experiences.
If you already have a 55-300 there's no reason to get a 50-200.
If you do get an M42 lens, make sure to get the original Pentax M42 to K mount adapter. It's pricey but others could jam up and you could have to use destructive force to remove the mounted lens from the body.
The A 50/1.2 is a bit of a n outlier in your list as it is indeed a very special and usually quite expensive lens. Have a look at the pictures that it is indeed the A 50/
1.2 lens and not an A 50 1:2, and that it is fine optically, mechanically and electronically.
I'd recommend looking for a 50mm lens with an aperture of 1.7/1.8. The K 55/1.8 is the oldest with K mount, then comes the M 50/1.7, then the A 50/1.7 offers electronically controlled aperture, the F and FA 50/1.7's offer autofocus to the mix just like the DA 50/1.8, but this one doesn't have a physical aperture ring anymore. You typically pay extra for the latest of those lenses, but you do get some features like newer coatings apart from the obvious one I listed above. The M 50/1.7 can be had for around $20 or less if you're lucky, but it's an all-manual lens, while the DA 50/1.8 will cost you at least 3 times as much (rough estimate), and the F and FA lenses are even more expensive used (you could get a new DA 50/1.8 for what some people want for their used F/FA lenses).