I've been riding for about 19 years and just got my first horse a year ago this November.
Willy's a "been there-done that" 16 year old Appendix qh gelding. He was packed on and roped off of, and has competed in western pleasure and had dressage lessons...he also carried kill after hunting trips, and so is quite seasoned with loud noises, and all animals.
I agree with M2G on this one. An older horse that can teach you a TON is your best bet. I learn so much from Willy every day. He's so patient and kind...but man, he lets me know if I'm doing something REALLY wrong !
Also, check out the potential horse's eye- a gentle eye is always a good thing to look for.