You don't need the enable aperture ring setting for the A series, just the previous lenses.
And it's not just the enable aperture ring, if you do buy older M or K lenses you also need to set up the green button to Tv Shift. It's all in that document that I linked to earlier.
The A28/2.8 and the A50/1.7 are fine lenses, I have both on my K20D and I especially enjoy the 28mm as a walkaround lens.
This is the 50/1.7 at 1.7:
Also, please don't mind my signature. I do love my Sears 1.7 but the Pentax-A is a very fine lens