The old 120/5,6 S-Planar is a very sharp lens. It is indeed the sharpest of the C-lens lineup. However, it is optimised for close up work and sharpness drops significantly when used at infinity. If used for objects within a range of a few metres, so as for portraits, the image quality is stunning. Technical reliability is like with all C-lenses. They are decades old and can have the typical problems (slow shutter, dried up lube), but if well cared for, there should be no issue. For portraits, I can recommend the 10 and 21 extension tube. They are really cheap nowadays. The only drawback with the 120/5,6 is that it is rather slow and the background blur is not as pronounced as with longer/faster lenses. From my own experience, having the 80mm, 120mm and 150mm lens is too narrow a spacing. I always found the 250mm too long. For only two portrait lenses, I would suggest the 80mm and the 150mm. The 120mm is a bit short for close portraits but okay for head and shoulder. My own preference is 80mm and 180mm by the way.