Originally posted by Kozlok One comment, the Tak55/1.8 goes for ~$40 to 50 as a rule, not 125. I use one with a $5 fotodiox adapter that I removed the locking clip from and screwed on tight to my lens. Works like a real K-mount lens. I think either the 55/1.8 or the M50/1.7 are your best bets at 50. For 20's, I'd save up another $200 and get the 21 limited (used) for ~$350.
I also was going to suggest the Tak 55mm F1.8 M42. And mention $125 is grossly overpriced. Look at ebay 'sold' listings (not the current 'asking' listings) and you will see many, many for $20 - $30 - those mostly for the older Super-Takumar version.
I would definitely go a bit more for the SMC version - $35 to $50 absolute tops. They are worth it for better contrast and flare resistance. (I have both, and while both are sharp, there is a definite improvement with the SMC version).
A word of caution if you enter the Wild West of ebay old Tak M42 lenses: 1. Avoid 1.8s that come with their Spotmatic body. There is a reason why they are being sold that way - either by someone who picked it up in a garage sale and usually knows nothing about it, or 2. the lens has defects - often fungus. The good lenses are removed and sold separately. The body is worthless and not worth the postage. Pay a bit more and get a lens being sold by an experienced seller - one who also offers a 14-day exchange. They are cheap, even bought this way.
If you don't want to put up with an adapter, the mentioned SMC Pentax-A 1.7 50mm at $50 tops is optically great (I have one, too). But the build quality is not the same as a Tak, and there is a common problem with frozen aperture rings (fixable if you get one really cheap)
Good Luck!