I am still in acquisition phase as far as prime lenses goes and I have not picked up a 70mm yet.
My only primes at the moment are FA35 f2, 40mm Limited, FA50 1.4, 100mm Macro WR, DA*300 - all of which I would highly recommend.
Check the review areas and search for threads on that lens in the SLR Lens forum to see what users think.
From what I hear I don't think you can go wrong with any of the Limiteds. Wouldn't want to use them unprotected in the snow and rain though.
Look at WR's or DA* lenses if you need weather resistance.
Let us know if you pull the trigger on that K-5