What kind of microscope for ML/biological research?
I agree with @Jeremias Brand's answer. 

Pretty much you will have to forget about fluorescence microscopy... you can probably find some dusty old one on eBay in your price range, but it probably won't be any good.

However, the good news is that seen that in your comment you mention 


  a) plants, b) blood, c) liquids such as wine, d) food? 


transmitted light (white light) would be perfect for a start.

eBay is surely a good place for finding used objectives at a cheap price. You may end up with a good collection of objectives for less than 100€.
That is surely tempting, and it may actually be OK as a start.
However, I personally think that it is better to own a good lower magnification objective than a cheap high-magnification one, so be always be a bit wary about offers that look too good to be true (especially for new piece of equipment).

I would strongly suggest getting a binocular microscope (so with two eyepieces), rather than a monocular one. Makes life much easier. 
Also, be sure to have a camera attachment. While a camera may not be strictly necessary, especially at the beginning, it's better to have an attachment so that you can add one in the future, if you feel the need of taking pictures. Although many microscopes have specialized attachments for microscope cameras, you can get away with using your own normal camera, or even your cellphone, if you position it correctly (may need some DIY depending on the situation). 
This page has very detailed explanations on how to connect a camera to a microscope.

As for your samples, plants are a very good and easy thing to start observing. Onions are probably one of the best samples to start with.

You may also want to stain your samples. Probably the easiest thing to get is methylene blue, which should be a few euros for 20-30 ml. However, fountain pen ink works as well.

Whenever you use chemicals, remember to look for their safety data sheet (MSDS). Methylene blue is pretty safe to work with, provided you don't swallow it (doh...) and use gloves (standard latex gloves are OK). Be careful because it stains and can be a bit messy, which is true for pen ink as well!

Crystals are also pretty cool to watch under a microscope: take a coverslip, put on a drop of sugary or salty water and wait for it to dry!