I go much more by breeding and what horse's close relative have been able to do. I gave up a long time ago trying to pick prospects by type.
For one thing, there is no measure of desire, heart, ability or the amount of 'pressure' a horse will take and still keep trying to comply. The point at which a horse that is asked for high level performance 'blows up' or 'gets mad' or just plain quits you determines how great or good or mediocre any horse is. This is why people pay huge prices for well bred prospects that come from great long lines of horses that have been or produced great performers.
Many years ago I was told by one of the greatest Reining trainers, Bob Loomis, that of all of the conformation traits you don't want in a reining prospect, you don't want a horse that is built 'down-hill'. You don't want a horse that is lower at his withers than at his hip. This was before he bought and trained Top Sail Cody (who was not built down-hill).
Well, a few years later a horse came along named Hollywood Dunit. Dunit and most of his get are anywhere from 1 to 4 inches lower at the withers (which many don't have) than they are at the hip. There went that great observation. The dunits were (and are) harder stoppers than most other reining lines.
Nowadays, no one will put the $25,000.00 plus that it takes to finish a reining horse unless that horse has impeccable bloodlines. Good prospects sell for $20,000.00 and more and the ones out of great mares and high winning stallions bring $100,000.00 or more.
Do you look at conformation? Of course! But, speed, agility, and innate ability (like cow) along with the desire and the ability to withstand a lot of pressure without blowing up mean a whole lot more.
Do mongrels and rescues ever make good horses. Sure. Once in a while a 'fluke' comes along, but you will go broke looking for them. If you want a good stock dog, you do not go out and buy a beagle.