A couple of points here just to note. The takumar 50/1.4 is an M42 screw mount so you need an adaptor to use it with your camera. No big deal but get an original Pentax adaptor for this lens.
The second point to consider is that the K10 and K20 do not meter well with legacy lenses, due to partially to the focusing screen they use, get an *istD screen. LL60 I think it is in place of the LL80 of the K20, or for manual focusing get a split image focusing screen
Here is how different cameras meter with a fast 50mm
Lastly, while Pentax coatings have always been good, in recent years every one else has caught up, so modern third party lenses while not equal to pentax's current coatings are perhaps better than older Pentax coatings. Technology has to some extent passed the lenses by. However, using a good and long hood on a lens, and protecting it from direct light can still yield excellent shots with the old beauties. Note however that the lenses from the K and M series lack the ED glass and complex lens profiles of modern lenses and therefore exhibit more lateral CA than newer designs. This does not make them in useable, but just be prepared for what you see as results