I would say to get everthing settled with your wife first. Make sure your wife is good with the concept. Then you should talk about who to approach.
For example, your wife may worry about ruining a friendship. Or she may be ok with being in a V relationship with you dating the friend.
As for bringing it up to your wife, you could do something from playing "what if" or you could email her the wikipedia article on polyamory and ask what she thinks about it. Ultiimately, you will need to be able to answer questions on jealousy and boundaries to her.