You pay a price for using lenses at full aperture--the images are
significantly less sharp than when you shoot at their sweet spots--which
typically are in the f/5.6--f/8 aperture range.

Leica makes lenses that are optimized for wide aperture settings, but the
Summicron 50/2 sells for about $1000 and the Summilux 50/1.4 sells for
nearly $2000.00, so I don't think you want to buy into that camera/lens
system.

If you buy a Pentax 50mm/1.4 you will get good sharpness at f/4 and
excellent sharpness at f/8.  One way to support using those smaller
apertures is to use fast film (although that introduces possible problems
with graininess).  Another way to compensate is to use slower shutter
speeds, and mount the camera in a tripod to avoid shake, but that assumes
that using a tripod is practical at the particular shooting location and
that the subject does not move.  I utilize tripods all the time for shooting
landscapes/cityscapes/architectural subjects, but they are all non-moving
and thus easy to photograph.  That would not be the case when shooting any
kind of moving or active subjects.

Check the link below to see what can be typically expected in terms of
sharpness at various aperture settings.  The lens in this example is a Carl
Zeiss, but the example can be applied to almost every other brand of normal
lens.  As you can see, the resolution drops of dramatically when the lens is
shot wide open.  If you are on a budget, the two most cost-effective lenses
for you would be the SMCP-A 50mm/1.7 purchased USED at KEH or eBay OR the
SMCP-A 50/2 purchased either new or used, at around $50.00.  Those lenses
were among the sharpest to begin with, so even at full aperture you can
expect relatively sharp results.  There is also a f/1.4 version, but that is
priced at over $100.00, and it is actually less-sharp than the other two
lenses (plus it has a bit of barrel distortion, as just about every f/1.4
lens does).

If absolute sharpness is your objective, then the 50/1.7 is the optimal
choice.  You might want to try to match it with a 200-speed film and see if
that combination gives you the low-light shooting capability that you want,
along with the sharpness that you need.

Here is the link for the f-stop comparison shots:

http://www.kenrockwell.com/nikon/50-comparison/f-stops.htm

Here is a link to a page that describes the subtle differences between
normal lenses of different maximum apertures.  The examples are of Nikon
lenses, but the principles apply equally to Pentax:

http://www.kenrockwell.com/nikon/35f14ais.htm