landscape -> wide angle, 12mm and above for dslr or 28mm for film
f-stop is no worry. u would want to shoot at f8 and above.
there are a lot of cheap lenses out there which do great landscape.
potrait -> looking in the range of 60mm - 200mm, get one with a wide aperture and gives good bokeh. common range would be the 85mm, 90mm, 100mm and the 105.
u are looking at 2 very differnet aspects of photography.
one is more studio the other is more travel... quite on the extremes