Any model will do! NB if you buy a Pi Zero, only the Zero+ has a camera connector, and even that needs a special lead. Of course, if you're using a camera that's not the Raspberry Pi camera module, you can use whatever USB-compatible device you like. Bear in mind that the holes in the microscope stand are designed for a Model B, so Model A or Zero won't fit correctly.
The Raspberry Pi will need a suitable 5v power supply, an SD card, and most likely a keyboard, monitor, and mouse to function correctly.