Hello I just got one of these and my dh is a bit of a expert in this area. He said its a good idea to get anti virus especially if buying stuff on it(of course I have!)Was lucky to get norton on offer for a tenner when bought the nexus. Hope this helps!
Norton anything is a waste of money, they don't pick up most viruses at all. The reason IOS is bullet proof is because all apps come via the Apple Store. If you stick to the Google play store, you'll be just as secure.