What microscope/magnification would I need to observe P. Acnes bacteria?
I agree with canadianer that chemical tests are generally more useful for identification.  However, morphology does help and some staining techniques can be useful for identification (the Gram stain is the obvious one).  If you are looking for a microscope, look for a nice scope with a 100x oil-immersion objective and a 10x ocular lens (12x would be OK too). Zeiss, Nikon and Olympus are some old names that make good scopes - and there are more.