OS X includes its own security systems as XProtect and Gatekeeper, so you don't need any additional security software which may make your computer slow or useless. Also, Windows viruses won't affect OS X, but if you will connect USB drives or external drives that you will use with PCs, I recommend you to install ClamXav just in case you transfer viruses to other computers.
If you want more information about OS X and malware, see > http://www.thesafemac.com/mmg
None - do not buy anything - Do not even consider Norton.
If you must have one - install Sophos or ClamX AV which are both free.
In any event, keeping your OS upto date is your best defense.