In any advertisement for horses available in Europe you will come across a number of abbreviations: KWPN, Hanoverian, Oldenburg, Danish Warmblood, PRE. These are the breed registries, and if you understand what they are you will avoid a great deal of confusion.
The main point that confuses most people is this: the European warmblood ‘breeds’ are not closed breeds like the Thoroughbred or the Arabian. Instead, they have open studbooks, meaning that they function as selection systems: breeding organisations which establish standards, examine and rate horses, and accept good genetics from any source, including from one another. The same successful stallion can therefore have registered offspring in more than one studbook. A Dutch KWPN horse and a German Hanoverian can be more closely related than two horses chosen from within the same breed. Hence, when an advertisement gives you the studbook, it is basically telling you which quality-control system the horse went through, rather than merely indicating anything about its genetic background.
Warmblood studbooks are in control of today’s competition dressage. The KWPN from the Netherlands, together with the German ones (those of Hanover, Oldenburg and Westphalia) and the Danish Warmbloods take the leading positions in the world rankings, each with its own particular character and reputation although all of them are aiming at producing the same type of modern sport horse; because they have so much blood in common, a purchaser is generally advised to assess the individual horse rather than worry about which studbook it comes from.
The Iberian horses are the only real exception; the Spanish PRE, usually known as the Andalusian, and the Portuguese Lusitano originate from closed studbooks and have been the subject of centuries of selection for collection: that is, the ability to carry weight when standing with the hindquarters extended, which is precisely the most difficult aspect of dressage. They are generally brave, friendly towards people, and easy to handle, which is the reason why they are so popular with novice riders. In exchange, however, they usually lose out in the long, extended trots that are rewarded in modern tests, although sport-oriented Iberian breeders have reduced that gap quite considerably.
Instead, buy a horse and let the studbook show you where to look and what to examine; the best horse for you will have come from each of the books mentioned here.