Far be it for me to try to talk you into being unhappy with your Pentaxes. They're top notch pieces of machinery. Fit, finish, and glass are impeccable. The 50/1.4 is a bit weak, but the 1.8 and 2.0/50's are terrific. Beats Nikkor IMO.
As for the guy wanting some Nikon gear, I'd say a 24mm, 35, 50/2.0 85, and 200 are all cheap enough, and great performers. Although I don't do AI myself.