It would depend on what your goal is..
Gain of a D7000:
Ability to use the older Auto focus lens (with the screw af mechanism)
Ability to meter old glass
improved Weather Protection
Improved build quality
The sensors are the same, so,, the picture quality will not be improved. I own both the 5100 and 7000, and so unless you are wanting to use the older glass, I am not sure what you will gain over the very fine 5100.
as far as value:
Used 5100 -> around 400 or so dollars
18-55 lens -> around 80 dollars (but I would keep this lens for the 7000, very sharp and handy).
So,, again decide if the upgrade is worth it,.