need advice on purchasing used microscopes

I am the volunteer coordinator for phytoplankton monitors. We
recently got a small sum of money (not much 1K) to purchase scopes for
volunteers.

We use them to identify diatoms and dinoflagellates in coastal waters.
I would like them to have a compound scope, with 10, 20, 40, and 10x
ocular; ideally, a mechanical stage.

Does anyone have any advice?
.