My favorite FF portrait lenses, back in the early 70's with my ES II, were the M42 SMC Takumar 105mm f2.8, and secondly (because it was so much heavier), the 85mm f1.8.
As I'm sure you are aware, the 85mm costs a fortune these days. But if you are persistent, you can find somewhat reasonable 105mms - particularly the non-SMC, which with its slightly less contrast still is great for portraits (use a hood though).
I almost always used the 105. Light. Great handling. Only when light was really a problem did I pull out the 85mm. (I also thought it was a tad short..) Shoot wide open, and yes, some say its a bit soft - but perfect for Portraits. At 2.8 and 105, the background quickly goes OOF, and the bokeh (not used term in those days) was creamy.
If the 105 can't be found for a reasonable price, maybe look at 135mm - though I do find them a bit flat for FF Portraits. The Tak M42 135mm f3.5 is a really fine lens - mint ones are less than $50. Also light, easy to handle. A surprisingly good cheap 135 is the Vivitar Komine (28xxx S/N) M42 f2.8. It is sharp enough wide open - possibly sharper than the Tak 135mm at 3.5, certainly at center. Mint ones are $30 to $40 - tops. The 8-blade iris (early) is best.
As to a 105mm Tak, unfortunately you can't do too much better on a 'Buy It Now' than this these days - $115 - supposedly 'near mint' -
http://www.ebay.com/itm/Asahi-Pentax-105mm-1-2-8-Super-Takumar-M42-Screw-Mou...item258c1eba3c. But if you persist at auctions you might pick one up for $90 or under...
Last edited by cahudson42; 04-27-2014 at 04:50 PM.