What are important factors to consider when choosing a Chihuahua handler?

Are you talking about a professional handler to show your dog at AKC events? Or a person to train your dog?

TRUST is the most important aspect in hiring anyone to work with your dog. You have to trust what they are telling you and that they can deliver on those promises. Never turn over care, custody, and control of your dog to anyone that you don't trust completely. Ask for references, and check them thoroughly. A reputable dog professional will not mind furnishing you with plenty of references.

Good question. Kudo's to you for thinking this through before making any decisions. Good luck!