As well as being a photographer, I work part time in a Camera Store and am familiar with these cameras. The D50 was aimed for the amateur market and is designed to give an excellent colour correct image etc. right out of the box without fiddling around.
There are several differences between the D50-D70/s and the main ones are:
1. D 70 has two command dials - very useful if you are shooting manual.
2. D 70 has faster frame rate - 3 fps vs 2.5 for the D50
3. D 70 has the 1005 pixel RGB colour meter ( this is the same as all of Nikon's pro cameras - the D50 doesn't have this meter.
4. D 70 has a top shutter speed of 1/8000 - D50's is 1/4000.
I would suggest that you go to www.dpreview.com
and put both of these cameras on the same page and you can compare their stats etc.
Normally the D50 in kit form would come with the 18-55 lens not the 28-80. The 28-80 is a lens that is designed for film whereas the 18-55 is designed for the digital bodies. I would strongly suggest that you look at the Nikon 18-70 Af-S Dx lens. I don't know what they cost in the US but optically and functionally this is an excellent lens that has been well reported on.
If you are looking at a used Nikon I would suggest the D70/D70s over the D100. The D100's technology is 3 - 4 years older that the D70/s and to my eye the D70/s takes a slightly better picture.