I'd suggest getting one that has a built-in motor so you have more lens options (also the lenses without the motors inside are cheaper). You'd be looking at the D50, D70, D90, etc. Used cameras I would avoid (because of the lacking motor) would be the D40, D40x, and D60.
You can pick up some really great used Nikon lenses for pretty decent prices. My favorite lens is my 50mm 1.8 prime that was only $100. Comparatively, the 50mm 1.8 w/ built in motor is $200
