Pentax K5, worth buying instead of nikon?

Sounds like the fa 43mm ltd would suite you. It takes unbelievable portraits. The bokeh & sharpness are a thing of perfection.
It's a little long for 'all purpose' use but once you get used to it, it's second nature. That lens is probably my favorite lens of all time(an I've shot with the best from C/N/S). But for dedicated portraits, the 77mm is where it's at.
I sound like a broken record, but if you buying into pentax, you should beg, borrow or steal to get the 31/43/77 combo. It's pentax's Crown Jewels imo.
(I'd also recommend getting made in Japan versions, but that's subjective).