The D90 is somewhat larger than the D40, but trying out strictly for 2-3 weeks will give you a preview of potential or possible purchases (in the near or far future) as your hobby and skill level grows. Yes "all in one" zooms do have draw backs including image quality (results are "okay" for the money) and build quality (because of the design; they prone to lens creep). They were created more for the consumer (who wanted to cover all the bases) than for the professional/freelancer.
The whole point of this lesson is to help you "see the world as you see it" thru the camera instead of buying lens after lens to "find one that works". As a freelancer I have to carry the "Holy Trinity" (14-24, 24-70, & 70-200) and couple of fast primes. But after years of shooting, if Nikon ever decide to make custom zooms; a 24-138 f2.8 or f4, that would be the zoom of my choice for me.