First define what the characteristics are you are looking for. There are definitely a few things that you always want to look for. You really want to trust your initial impression of their attitude and character which is not always easy to test in one or two days of trying horses. However, you can usually tell if there is something very apparent right away. In addition to attitude, scope and talent is very important. We like to see a horse that respects the rails and is careful.
Everyobdy has a type ... so define what is the 'type' of horse you'll be looking for. Wome really like a horse to have a lot of blood others like them more cold blooded.
Is age important? Age is definitely secondary. If you are working within a specific budget and have clients that have specific needs than you should look on the younger side just because the older you get the more expensive the horse. But in general it should be more about type and quality than age.
Of course you cant to ask some questions to the seller. But you don't want to ask them to many questions. You want to be able to get a feel for the horse on your own and judge what you see. In today’s world it is also very easy to get history on the horses so it is hard to keep anything secret. Everything is very transparent. You’d rather have your own opinion of the horse than get influenced by the person that you're asking.
And the same old question... Does size matter? Generally speaking no. In general, smaller horses tend to have more blood and are a bit quicker and hotter than the bigger, slower moving horses.