Just to clarify...it's not true that only D90 and above models have dedicated buttons to access ISO/top lcd/in-built AF screws...lower older models like d70/d80 have it. Just that you can probably not find new d70/d80 to buy any more.
D50, D70, D70s & D80 are predecessor of D90, which is now a predecessor for D7000 (logically). These models should be regarded as the same level, even though the later outperforms the older model. They all share this common attribute of having in-body motor, and some other features such as bracketing function (with 1 exception model now).
When we say lower end, we usually mean models like D40, D40x, D60, D3000, D3100, D5000 & D5100. These are the models that does not contain in-body AF motor.
Just to give you a full picture, higher end bodies, or to say the pro series, has even more dedicated buttons, such as D200, D300, D300s, D700, D2H, D2X, D3 and D3s (there are others which I"m not sure if I should include in discussion). Not so good in history but I believe all these models should have in-body AF motor.