With all due respect: why are you limiting yourself to the under 10 age group? Maybe your perfect horse is 11 or 12 and just waiting for you to show up.
Honestly, I think that you're focusing on the second-tier incidentals - height and age - rather than the first-tier attributes like discipline, mind, training level. Getting caught up in color, height, breed, papers, gender - those are beginner issues. What you need to concentrate on are intangibles rather than superficialities.
The same is true of humans: we cannot control the year in which we were born or the color of our eyes or whether or not we were born into aristocracy. What we can control are the important things - like attitude, willingness, experience, etc.; and those, IMO, are the most important attributes to consider when purchasing a horse (or finding a lifetime partner).
Just my two cents.