Have you had any experience with microscopes?
Do you have a budget in mind?
You should figure out what you want to take pictures of and under what conditions.
Microscopes are often divided into two kinds:
1) a
compound scope with different powers of lenses that can be switch around. It is usually used with microscope slides. The lenses often have a very shallow depth of field. This means you have to focus up and down to get a good look at a large 3D object. Microscope slide are usually very thin slices of something that used to be alive. To view something alive and moving (like a Daphina or amphipod) you will have to get (or make) some special slides so your little animals won't get crushed.
Tinier things like paramecia will usually be fine under a cover slip, but you might want to prop the coverslip up on pieces of coverslip to hold it up, so things aren't crushed.
2) the other kind of microscope is a
dissecting scope. It is a lower magnification scope, with a longer working distance (distance between the lens and the object in focus), and a much bigger field of view (how much area you can see). This kind of scope is good for manipulating things. For example, they can be used for surgeries.
Its my impression that now-a-days you can get a pretty good home microscope with a digital camera built in, at a decent price.
You might be able to get a nice used compound scope from people who were in vet or medical schools for a bit more.
Research grade scopes cost much more.
If you are seriously shopping for a scope, it would be good to try it our with some samples, of whatever you are going to want to look at, before you buy it, if possible.
Something like the FB microscopy group
@DaveE mentioned would be a good source of information about what is out there now, that is good. To some extent, it would be a stand in for really checking it out in person.
Lighting is also an important aspect of microscopy.