Most any turkey fryer burners like the one Ed pictured are going to be at least 50K BTUs or more, and some range up to 170K BTUs. But a 50K to 70K burner will do fine.
Just make sure you get one with a sturdy stand that will very securly hold your brew pot full of wort. I've got an old Bayou Classic that is square shaped and made of heavy steel. It's about 19 or 20 inches wide and is extremely stable. I have no idea how many BTUs, but it boils my water or wort pretty fast, and I never even open it up more than half. I think stability is more important that BTUs, since any of these fryers pack enough heat to do the job.