Being a "Unicorn hunter" isn't right or wrong - it's what you want. The reality is that lots of folks have come on poly fora like this one, bemoaning the fact that they can't find anyone who is suitable - if you look around on here you will find several recent posts.
The main issue is that by having the stipulations that you have, you are are severely limiting the number of candidates for you - by needing her to be single, and enforcing the rule that she have a relationship with the both of you, you reduce the number of females that will even consider it.
Let me give you a few more things to think about, none of these are "tests" or right/wrong answers - they're here to help you sort out what you are looking for:
* Are you going to keep on doing things with your current partner alone once she is in your life?
* Are you going to allow her to go out on dates with just him. is he going to allow you to go out on dates with just you, or do things have to be done as a 3?
* What about sex? Must it always be the three of you? Or is it ok for you to be with your current partner, but not her alone with either of you?
* Is she going to live with you, or is she expected to keep her own place?
* What about fluid bonding? How would that work between you?
* Is this primarily about sex between you, or are you thinking of this as a full relationship?
* What about children? If she is single but has children, is that ok? What role would the two of you play with those children?
* If the relationships between her and each of you progress at different paces (she feels more for you than him, for example) - is this going to be a show-stopper problem or are you ok with letting each develop?
* Is she allowed to go out on dates (and maybe start a relationship with another person), once you are in this relationship? Or are you expecting this to be completely "closed".
* If you find this person, would the two of you not be looking for any other relationships (sexual or romantic) outside of the three of you?
These are all things that a prospective unicorn would be asking of you, so it's probably best to have those answers discussed between the two of you and ready. There may be more, but those are what I can think of off the top of my head.
"Listen, or your tongue will make you deaf." - Native American Proverb