I disagree with telling her at all. What do you actually hope to accomplish? Even if she reciprocates you're leaving for a new job anyways which is further away from her. You had 2 years to make that decision and that move, and now your time is just reaching the end and you've decided to take an "All or Nothing" approach.
She is in a relationship that is close to home, she sounds like she is happy and it is working. I think it would be unfair to ask her to give that up for a long-distance relationship with you where there isn't even a guarantee of basic compatibility. Sorry man, but this is a case where I believe the most fair action is to cancel the lunch and just move away. In time you will forget her.
If you must keep things alive, then I'd suggest keeping her on Facebook or get her number and just keep in touch, if things fall apart with her current bf, or you decide to move back, then you might be more in line with making a real move, but really, it is just a scumbag thing for making a move on a girl in a relationship, especially if you respect her (and therefore respect her choices).
"All is fair in love and war." - Francis Edward Smedley