Do you need a media planning and buying agency?
Technically, not at all. You can, and other brands have done so, go it alone. You will, in the short term at least, save money that can be spun back into creative or the service/product itself.
Having said that, the chances of running a successful media planning and buying campaign (and resultant ad campaign) without expertise and experience are slim. Selecting a good agency means you will be far more likely to reach your target audience with your brand and campaigns. Professional media planners are the foundation on which most successful advertising campaigns run. They know more about the audience and the media landscape than anyone else. A good agency will research and plan media, locate and negotiate suitable media space and provide holistic justification for why and how you should spend your money.
You get better performing campaigns and a better performing brand through a good media planning and buying agency. It is the job of the marketing department to put a price on that and weigh it off against the money you could save by doing things in-house.
What do you want/expect from a media planning and buying agency?
Make a list of what you hope to achieve from a partnership with an agency. For example, what can be expected in the way of communication? What sort of rationale will be provided for the decisions the agency makes on your behalf? Do agencies think your campaign objectives are realistic?
Not all agencies and clients are a good fit. For example, some might have more experience with charitable organisations while others might be more suitable for very corporate entities.
An honest relationship with an agency is worth its weight in gold to any organisation.
Would it be better to choose a full-service advertising agency that also offers media planning and buying expertise?
Media planning and buying are just two aspects of advertising. A full-service marketing agency can provide more aspects – PR or ad production for example.
Specialist agencies tend to represent better value because the client isn’t also paying for services (and overheads) they don’t use – as is the case when larger agencies are employed.
Smaller agencies are often the experts in their field. Size is no guarantee either way (there are of course excellent large full-service agencies), but at the very least you will be paying more, for no appreciable increase in quality of work.