Instead of 105mm, would it be safer to go with a 75mm lens?
The only lens on my pentax that I own is the standard 50mm.
What lens are suggested?
Please note that K-mount lenses are easy to find. Many are the same or less than the old Takumars. For example, my SMC Pentax-M 80-200/4.5 zoom cost $50 on ebay. My SMC Pentax-M 28/2.8 cost $35 from KEH.
If you want to add lenses I recommend buying K-mount lenses. They will have automatic aperture closing and full aperture metering. Adapting Takumars will mean that you will will have to close the aperture manually each time. And if you want to mount your 50, you'll have to remove the adapter first.
I use the adapter with the Takumars and other screw thread lenses for the specific qualities those lenses offer, but still have the equivalent K-mounts for regular use.
Which additional lenses is something you'll have to figure out, though general guidelines can be given. Do you need a little wider view? A 35mm could be just right. Do you want a considerably wider view? Try a 28mm.
For longer lenses, a 100 will get you twice the image size as a 50 for any given distance. Lots of people prefer a 135mm though, for the extra reach, while still having an easy to handle lens.
Another way to go is a short zoom. A 75-150 zoom will give you a good range to work with, and the Pentax one is quite good, reasonably priced.
I do use a digital device in my photographic pursuits when necessary.
When someone rags on me for using film, I use a middle digit, upraised.
Perfect advice. I only need to narrow down my view just slightly to get one standard image onto focus. I was going to go with 75 or 100/105 in order to narrow down the lens size gradually, instead of overestimating....