I've been thinking a lot about microscopes for naturalists lately.
Especially when first exploring wildlife in close up, a good low to medium power stereomicroscope is hard to beat. Useful magnifications are about x7 - x40 (or greater). Most stereo microscopes offer more than one magnification, either in set steps, or as a zoom. From a naturalist's point of view, this type of microscope is useful for examining insects and other mini-beasts, looking at fine details of plants (including mosses) and examining bone and other remains, for instance in owl pellets.
As a general rule, about £250 will buy you a decent microscope (with occasional bargains for less second hand).
I was recently asked to source a decent second hand microscope for a friend, who wanted one for his grandson (who is seven, so needed something easy to use). I found a Leica Zoom 2000, barely used, for about half the going rate on ebay.
It has a zoom range of x7 - x30, the option of lighting from above or below, or both. Robust, being designed for student use (so expected to withstand abuse!), and simple to operate.
I also just bought this Zeiss Stereomicroscope. It does much the same job as the Leica Zoom 2000 above, but is slightly more difficult to use, and so would suit an older student.
One of the advantages of this microscope (greater choice of lighting position) over the Leica is also one of its disadvantages. The light is tungsten, and can get hot if left on. I've thought about taking the bulb out, and replacing it with a cheap LED torch of the right diameter.
It has a magnification of x6 - x40 with the current x10 objectives, with steps of x6, x10, x16, x25 and x40.