The earlier f/2.5 version of the 90mm Tamron is a very well respected lens, but only has magnification ability (natively) of 1:2 .. ie. not true 1:1 macro.
So your small objects won't fill the frame as well as a true 1:1 macro lens will.
It's still a great lens apparently. I don't know how good the AF version is, but I know that they are quite slow and clunky.
The original 90mm f/2.8 version of the Tamron was screw driven on Nikon bodies, but the later model had a change of focusing drive to a lens mounted motorised system.
(this was primarily for the cheaper AF-S only Nikon bodies, below the level of the D90/D7000).
I have no idea if this new in lens af drive is better or worse than the original lens, but the original lens focuses well enough if a bit slow.
Note that almost all AF macro lenses may not focus as fast as an equivalent non macro lens of similar type.
Macro lenses tend to have 'more accurate' focus gearing, where the gearing requires more turns to move the lens elements the same distance. This is turn dictates af speed!
There are many features available to you on a D7000 compared to the likes of a D3200(cheaper end body) that you could use to advantage once you learn how to use them.
as an example: D7000's CLS capability, which simply means that you can use a Nikon(or even other brand) external flash remotely and wirelessly, whereas with the cheaper camera body, you'd have to invest in (sometimes clumsy) accessories to achieve the same result.
In many macro situations, the ability to have flexible flash setup can mean getting the shot or not.
Yeah you can use an external flash on the camera body .. but once you experience the Nikon wireless system, you may also love it. It just gives you options not otherwise available to you.
I would forego more megapixels for more 'real features' any day of the week in a camera body.