Don't suppose you have tried googling for microscopes and getting an idea of price that way?
Presumably you want to observe the micro-organisms on a slide, with a coverslip over the top to make a wet prep. That is probably the best way to observe the parasites and other microscopic pond life you want to demonstrate. Just looking myself, I saw a few going fairly cheap, and cover the sort of magnification range you would need, with objective lenses ranging from x4 to x40, with a x10 ocular. Therefore, you would have x40 to x400 magnification which would be sufficient, certainly in human disease, I can identify parasites reliably up to x400 magnification. By the looks of things, Daphnia is actually rather big, so you may not even need a x40 objective, though it will certainly give you a nice "up close and personal" view with plant and animal cells (eg: onion cells, or the epithelial cells from the mouth).
Anyway, I am not to sure about the quality of the lenses though, and how well it will provide contrast for the observation of amoeba and other similar organisms. In other words, I can not really give you any recommendations, the microscopes I use for work cost in the order of US$5000 - $8000, however they have a few extra odds and ends to them which mess around with the emitting light.
Also, if you do find yourself a decently priced microscope, don't rush in with the added option of buying TV/external monitor connections. What you see on webpage advertising vs what you might get in real life are two different things. If you can get a real world example of the image capture, then that would be best, considering many images picked up by a fixed camera can be washed out due to the light from the lamp, or, if the lamp is dulled, be unable to see any real detail. Just think of what happens when you point a camera at the sun, or take a photo in a darkened room.
Plus, I thought 40th birthdays were all about fast cars, bungee jumping, sky diving, and base jumping adventures?