Microscope buying help?

If you get something with the range 30 to 100 to 300 or 50 150 300 that is great for most people. Once you go over 300 you are into staining, and gram positive and negative and oil lenses and it gets dramatically more difficult. As in poking the objective through the slides. Everything gets harder actually. You need thinner sections, you need better light, you need finer focusing. You need to clean the lenses more often and more carefully.
Its a pain.
If you just want to look at leaves and living rotifers and waterfleas and protozoa, then 300 is plenty as your max. Mirror and top and bottom light are good to have. Mirrors can be adjusted to get great shadowing or perspective at the low magnifications.
Actually I got a magnifier for the computer that plugs in. If you have a windows netbook, that might be an option for field trips. BUT you need to make sure it has the full range of magnifications. I did not notice in time in mine. It is good at the 50 magnification but then it fades out until about 250.
It takes videos and pictures and has bright top lighting.
When you think about it, something like that should be a camera that you can take on field trips and that records to a sd card.
My magnifier is a windows only thing. I tried it in wine on linux but no go.
Hope that helps.
Brian

1. It depends what you plan on looking at.  Between your eyepiece and your objective lenses, aim for a magnification range of around x40 to x300 or x400.
2. Will you always use the 'scope in a lab, near a convenient power-point? Get a light. Is there a chance that the 'scope will be used outside the lab? Get a mirror (if you end up using the scope in a dark corner, you can supplement the mirror with a small desk lamp).   However, illumination from above the sample can be useful if you are looking at larger specimens, such as live insects or leaves.
3. Iris or regular makes no real difference to the operation.  Iris is easier, but more expensive.  Choose by the depth of your pockets.
4. Yes.  Especially with a very shortcrust pastry.

Built in light.

10 or 15x
the lower magnification/more expensive one.

diaphragm? Whatever. An adjustable mechanical stage is more important.

pie? Depends on the pie.