It's certainly not essential to employ the services of a Spanish estate agent when buying property in Spain though of course there are many advantages in doing so. An agent has a ready stock of properties, will accompany you on visits and will give you the benefit of their often valuable advice. On the other hand competition is extremely fierce among agents in Spain so the sales techniques are sometimes extremely high pressure and the commissions can be exorbitant.
Beware of the prolific property exhibitions in the UK and other north European countries because the information given about the properties available at certain prices can be misleading, if not downright inaccurate. Glossy brochures and posters paint tempting pictures of luxury villas which you're told you can buy for less than the price of a two-bed semi in England. You book an all expenses paid inspection trip to Spain only to find there are no luxury villas in your price range because "the last one's just been sold". Some unscrupulous developers and agents use these techniques to get you out to Spain where they hope they'll be able to "persuade" you into buying a property that is completely different (and more expensive) than those you salivated over at the exhibition.
If you want to use an agent your best bet is to make appointments with at least three agents in your chosen area, preferably in advance of your visit to Spain (most good agents have web sites these days with their contact details). Take as much time as you possibly can to see the full range of properties available and NEVER make an offer after your first viewing, however much you love the house. Take copious notes, ideally take some photos as well, then go away and think up some pertinent questions to ask on your return visit. Visit the property at different times of the day so you know how much sunlight it gets (or doesn't!) both in the morning and evenings.
Be warned that some agents charge 10% or more commission on top of the asking price and that figure is carried by you, the buyer. The agents justify these high fees on the grounds that they're generally dealing with both a foreign vendor and buyer who need a lot more help and guidance than in the case of two individuals negotiating a sale in their own country.
Estate agents don't need any formal qualifications to operate in Spain so it's a good idea to seek out one that's a member of a professional body. Ask to see proof that they are either a registered API ("agente de la propiedad inmobiliaria" or property agent) or a "GIPE" ("gestor intermediario de promociones and edificiones" which means property development and buildings representative).
If you have the time and energy you can seek out properties that are being sold directly by the owner without an agent, so saving yourself thousands of pounds worth of commission. Many owners advertise their properties for sale in English language newspapers in Spain, on notice boards in bars, clubs and supermarkets and via "se vende" (for sale) signs on walls and gates. There's no harm in by-passing the agents as long as you employ a good lawyer to protect your interests.