I use a Canon PowerShot SX400 IS that my wife got from a friend while I was in residency (for $60), and it works great for my purposes (including when I went through my Oral Boards). I agree with droliver otherwise!
What helps is to have a standardized setup in your office. I use a 'clinical' blue background for my before and afters, and the floor has markings for the patient to orient him- or herself, the same way, each and every time.
The ASPS puts out photography guidelines/standards which are helpful, or else you can pull 'photography for plastic surgeon' articles in our literature, including Plastic and Reconstructive Surgery (PRS).