Pentax lenses for macro: every Takumar, SMC (K), M, or A in 50 or 100mm would be ok.
Cheapest would probably be the Takumar 4/50 Macro, most expensive (and rare) the A 2.8/100 macro.
But they are all very good, and they are all only down to 2:1 on 24x36 (a bit better on APS-C, fortunately).
Pentax lenses for low light: SMC Pentax-A 1.4/50. For $$$ of course the 1.2/50, but difficult to find.
For portrait: the 1.4/50 would be ok for that, too. If you want it a bit longer, it gets more difficult. The classical length on 24x36 of 85mm is sometimes a bit long for APS-C, and the ones with small DOF are getting more expensive every month, no matter whether a Pentax 85mm or third party (you are probably 2-3 years too late).
For landscape: Not so easy. The A 2.8/28 is good, the Takumar 3.5/28 (m42) sharper. But these are only "normal" length. Shorter, but much more expensive any 24mm Pentax. These focal lengths weren't so popular at that time, and very expensive, and many 3rd party lenses far from today's standard in IQ. I think you should have a look at our lens data base for reviews.
For real short lenses I am afraid you'll have to stick with rather newer lenses designed for APS-C.
EDIT:
For portrait, I am also using a Russian Helios 44K-4 (PK bayonet). This is 2.0/58; I got it for nearly nothing from a Russian immigrant together with a broken Russian SLR, but it looked really unused. There is a lot of copy variation with these lenses, but mine has fantastic centre sharpness. The resolution of my 10MP sensor (K200D) is the limiting factor, so I cannot compare with my Pentax lenses.
Last edited by RKKS08; 07-14-2012 at 03:19 PM.