i know next to nowt about microscopes but i would imagine that as with other optical stuff like cameras and scopes if you only have a limited budget you would be better off buying a decent model second hand rather than a cheap new one.
looking on the web it appears that there are a few sites doing second hand m/scopes but like i said i dont know enough to comment on whether these are a good deal or suitable for your needs
one such is
Colco Scientific: Secondhand Microscopes so perhaps one of the more knowledgable folk could have a look and tell us whether these are indeed value for money.
(NB: although this is a link to a commercial site I am in no way advertising, promoting or recomending them - it is only an example for the purposes of information)