You need at least 100x and good resolution. As far as necessities go, that's about it. 400x is a definite bonus.
My microscope gives me 40x, 100x, 400x and 1,000x(oil immersion) with the 10x eyepieces and with the 16x eyepieces I can get up to 1,600x (oil immersion). I only use the very high magnifications if I'm looking for something like Cryptosporidium. Due to the specialized staining techniques involved and the expense I doubt if you will ever have the need to do that so the oil lens is just not a necessity. I also have a mechanical stage which makes methodical scanning of the slides much easier and a binocular head which keeps eystrain and headache from setting in after several hours of looking through the scope.
As far as flotation medium, I use Fecasol. I also use Fecalizer kits to prepare the float. It helps to stain the specimen with Lugol's Solution. I use 5 drops of Lugol's to a Fecalyzer.
Identifying the things you find is going to take some practice. The charts and books are very helpful, but in the beginning, things are not going to look just like they do in the books and charts every time. Some eggs look amazingly like others...you have to first learn to use the microscope (control lighting) to get the sharpest, clearest possible image (much more difficult in the beginning than it sounds), then learn to see subtle differences in the morphology of whatever you are looking at.
Bacteria are a whole different challenge...you will need to learn to use differential staining techniques such as Gram staining, Acid-fast, etc. Then you will need to learn to identify what you find. Sometimes the best you can do is to say, "well, it's a Gram negative rod." and treat accordingly because (fortunately) antibiotics and antimicrobials tend to be targeted to specific types of bacteria.
