This is strictly our opinion and other breeders may have their own criteria.
While the Spanish Mustang has a number of different head profiles, we personally prefer the convex head with a fairly refined muzzle because we feel it is the most indicative of its Spanish ancestors. The best example of this is the Sorraia horse, indigenous to both Spain and Portugal. We like to see medium length straight ears for the same reason.
We want a neck that is deeper at the base than modern breeds and some horses will also have a pretty good crest. The neck should blend well into the withers and should as well have a well laid in sloping shoulder. We want the withers to be higher than the croup. This is an uphill horse. We also want a deep narrow chest, v' ed up nicely in front and a short to medium back. We prefer a fairly round croup although some horses are flatter and we really like to see a low tail set. By this we don't mean the position of the tail, but that the root of the tail fits smoothly into the hind quarters. Flat smooth muscling with no bulging or creasing. To make this point clear, have you ever seen a mule with bulging muscles? Mules are known for endurance as are Spanish Mustangs.