Any first DSLR is always a stepping stone onto the creative world of Photography and in reality, even a very basic DSLR, will be great for learning/growing and can last a lifetime of usage, or as long as the electronics and mechanics holdup. Time for change generally comes when the photographer has increased his/her ability to be creative and they find limitations with their current camera. There is no time limit to this and can happen within a few months to years or never, where some people even go back to smaller cameras! Cost effectiveness also depends on the buyers budget and specific requirements from that budget.
Due to the nature of current DSLR’s continually being superseded with newer models, it is difficult to be specific with regards to any particular model and also provide a specific answer to a shopping recommendation and therefore, a good starting point will be to watch a “hands on Video” on Youtube.
Regarding the Lenses; Nikon lenses are all F mount, however, some lenses (DX - covering 24x16mm of the DX Format) are designed for Crop Sensor sensor cameras and others (FX - covering 36x24mm of a 35mm format) are designed for Full frame sensors. All lenses will fit either sensor size, but the DX lenses fitted on a full frame Nikon may provide extreme cropping and vignetting and not be able to take full advantage of the full frame with a DX Lens.
