I'm surprised that no one mentioned this, but you could get a good sample of a Jupiter-3, send it to Brian Sweeney to have him shim it and do his close-focus mod then you'd have a killer Sonnar lens with a fast f/1.5 aperture and focusing less than a meter. You can buy 10 of these for the cost of a user Summilux.
If you want to stick with a Summicron ever, the rigid and dual range are much lower contrast but higher resolution than the later 6 element 'Crons. I have both and I'm looking for another, actually because I love the early 'Crons. There IS a Throium glass collapsible Summicron, very much like your 50 Takumar. Similar computation, and similar glass. Those are great performers and would probably show the softness you want.
If you think you want a Summilux, then get a Leica screwmount 50mm f/1.4 Nikkor. They have a close focus range that will bring your subject to within about .5 meters. Closer than any Leica lens aside from the DR Summicron. This plus the wide aperture and the fact that the Nikkor is just killer at close range will help to give you the look you want. See Dante Stella's site for a great review of some alternates to Leica glass.
The Canon 50mm f/1.2 is big and it has a lot of the aberrations that make for a soft image like you are looking for but it also is not nearly as well corrected as your Takumar and so displays a lot of things unpleasantly. The background can get "confusing" instead of smooth. The Canon 50mm f/1.4 is a better lens and is 95% as good as the pre-ASPH Summilux at 1/4 the cost or less.
There are a good number of other fantastic optics that can give you that lower contrast look without breaking the bank. Actually, for the cost of a 50 Summilux, you could buy a J-3, J-8, Canon 50/1.4 AND a Leica 50/3.5 Elmar. Try out all of them and see which you like then sell the others. The 50/3.5 Elmar produces a sharp but not too sharp, lower contrast image. Each one is different. That's the great thing about 50mm lenses and about RF photogrpahy. These lenses and formulations have been around for so long that we now have a "recipe book" of what lenses give what characteristics at what apertures and distances. It's amazing. You could have 30 different 50mm lenses that will fit a Leica M body and all could render differently. You can do the same with your Pentax and the various M42 50mm lenses out there. There are indeed some jewels.
Don't fret, have fun.
Phil Forrest