Oren, I agree with Mike: all screw mount Takumars are excellent lenses. What you get with the different models is more like flavoring -mostly- though formulas did change slightly over time. Pretty much, you have a progression from oldest to newest: Auto -> Super -> Super Multi Coated (S-M-C) -> SMC. S-M-C and SMC woudn't have much difference optically, SMC has an extra feeler in the mount for the full aperture metering of contemporary Spotmatics. The main effect of this is one might think the lens is broken because you can't switch from Auto to Manual when the lens isn't mounted. Otherwise the differences have to do with coatings and cosmetics. There's nothing wrong with the Auto and Super, though they tend to cost less than the multi coated ones.
The other thing to look for is yellowing due to radioactive glass. Not a big deal for me, I actually like the effect. Anyway there's a lot written on the topic.
Here's a nice summary of the Takumars:
Die Cast Pro - Pentax Lenses
Here's a pic of a few 50's:

You can see the older of the 50's, the Super, is more yellowed than the SMC. The 55 isn't yellowed at all (and is a fantastic lens by the way, sharper and less prone to CA than the 50)
The Pentax adaptor is under the Limited.
Here's the Super 50/1.4 on a Program Plus - Yes, I use the Takumars on my K mount film cameras too! They work even better on the Pgm Plus than the K100D - the exposure in auto works great.