"portraits" mean different things to different people. Close-up headshot, environmental portraits with a lot of the scene involved, full-body and torso...the options are endless.
Conventionally - which really doesn't mean anything - portraits are not required to be super sharp. Softer images are said to be more flattering. personally I think that's a bowl of hogwash, but some people swear by it.
Any prime is sharp...35mm AF-S, 50mm AF-D, 85mm AF-D, 135mm are all somewhat affordable. The 50mm AF-D has been produced since 1986 and they are still making new ones.
Some portrait photographers swear by a zoom..like the 70-200mm F2.8 VR (I or II). They like the distance-to-subject it affords them and from a distance the proportions between standout facial features like nose, ears etc. is more flat.
Personally I like to get real close..in my subject's faces...because it creates something. It is interaction and my presence influence the image. Sometimes for the worse, but most of the time for the better. But that's just me.
From a color-perspective, many swear by the Fuji S5 Pro and the super-CCD with it's larger and smaller photo sites beside each other, it's said to reproduce great skin-tones. If you can find a used one, you can spend more of your budget on Nikon lenses (S5 is a D200 body with a Nikon lens-mount).
Others swear by the three-layer Sigma FOVEON sensor, also especially because of it's rendition of skin colors. However the SD15 is expensive and you can't really find any SD14s used.
Personally I'm reasonably happy with my D300, but I always felt the D40 did better in the portrait-department due to it's color-rendition.
If you're just starting out, I recommend you buy just one lens - a prime - and practice a lot - before you invest in expensive zooms and whatnot. Furthermore, portraiture may well demand more of you in terms of portable controllable lighting and understanding how to create flattering light is probably more important for portraiture than body and lens put together.
Good luck with your choices!!