I dont think she based it on anything in particular, it probably just came to her naturally that was what Edwards name would be. Although in Irish, the word Cuilinn (anglicised to Cullen) means roughly "handsome boy." Not sure if this is linked, but its interesting to note :]
I thought she said the surnames as well as most of her ideas came to her naturally. The only things I've heard the surname "Cullen" in was Alex Cullen from the Devil's Advocate and the said to be power witch family Cullens from waaaay back in the day.