Wilt, please read the Ken Rockwell article on Nikon lens compatibility linked above. Your information is not correct.
1. Virtually all Nikon digital SLRs have mechanical control of the aperture so you can view at full aperture and the camera stops the lens down when the shutter is fired. That is, they all have the mechanical stop down lever inside the mount, even the cheap entry level DSLRs. However, the lower-end DSLRs do not have an AI indexing tab, so you don't get metering, and in general you should not mount a non-AI lens on a DSLR body, because it might foul some of the body mechanics (just like you shouldn't mount a non-AI lens on an Nikon AF film SLR).
2. There are Nikon digital SLRs that have the AI indexing tab around the edge of the lens mount so they can meter with a manual focus AI lens. Not only the Df. These are generally the "pro" (D2, D3, ...) and semi-pro level bodies (D200, D300, D700, ...). This is actually really useful, and the older D200, D300, etc bodies are fairly inexpensive on the used market.