I have some "ideals" for both the Thoroughbred and the Quarter horse (the breeds I am most familiar with).
These ideals have physical features that make them long lived for the job at hand and the brains and attitude to "git 'er done!" I want a Thoroughbred that can run fast but is physically substantive enough to do the other jobs they do well such as Eventing and Grand Prix jumping.. Dressage is in there too.
Quarter horses need to look like they can run very fast for that quarter mile or herd and work cattle.
Both these horses need good minds and good bodies.
Good luck finding that good horse. They are out there. I got one years ago for $1500 as a two year old... darn near perfect.