He will already have a duplicate range situation. His D3100 has the 18-55 and 18-140 lenses, but those are DX lenses, so he would need a different lens to cover the "normal lens" range on the FX D750. Maybe the 24-120. If you do not keep track of what lenses are DX vs FX, you can get mixed up, and get a DX lens rather than a FX lens. Example, there are several 70-300 lenses, of which I think ONE is a DX lens. Worse, I think that DX lens is a P lens, which won't work on the 3100 nor 7100. So he has to make sure to get a FX lens that will work with the older cameras. Actually for around the house shooting, I would go with the 3100, because it is lighter and more likely to be used more often. I speak from having a F2 and not taking it out cuz it was a hassle to pull out the "big" camera for grab shots. And today of using my P&S rather than my D7200, cuz the P&S is lighter and easier to use for grab shots.