I'm going to hook up with a friend after turning him down
What made you change your mind?
I thought I wanted to just keep him as a friend, but more may be good!
It is okay if you find a friend attractive.  It is also okay to grow a friendship into a relationship.
I'm not sure if that is a great idea.
There are a lot of relationships that stem from long term friendships. Taking the chance to try to date a friend is a big one, but if you both think it will work, give it a try.