The Brock microscope is the one microscope that I DO NOT recommend. Focusing is very tricky. Then, when you wish to go from scanning to higher power you must disassemble, screw in the higher power lens, and refocus. Refocusing under the higher power is almost impossible.
Therefore, go with the three-lens, par focal, standard student scope, an example of which may be seen at
They are actually less expensive than the Brock.