No, however it also depends on what type of use you intend the drive to be used for. If you plan on just a Time Machine backup drive then any USB EHD will be fine however I'd stay away from Western Digital products as their EHD's have a high failure rate.
Yes, if you want to be able to hook up the hard drive straight out of the box and start copying files to it. Go to amazon.com and enter "external hard drive mac" as your search words then read the lowest scoring owner comments of each model you are considering buying to see whether or not owners said the hard drive worked straight out of the box. Some do not and require "formatting" which is very confusing to a beginner especially because the instructions for formatting are explained in geek-speak instead of plain english.Do I need to buy a special external Hard Drive for my Mac?