I guess the first thing to decide is what type of image do you want? Is there a particular style you prefer? How big do you want your final image/s to be?
Next is the quality of the image, are you happy with something straight out of the camera or would you prefer someone to spend time editing the image/s to ensure a stylish professional finish. Let's be honest, there is nothing worse than a beautiful picture spoilt because the horse has an eye bogey or the owner has a mobile phone sticking out of their pocket!!!
Take your time, do your research and look at lots of different photographers. What do they offer? Are they insured? What training have they had? Do they specialise in the type of photography you prefer? What type of equipment do they use - the bigger the final image the better quality camera etc will be required. Alot of cameras these days are pin sharp until the weather is a little bit dull or overcast and then they really struggle to compete with a professional set up - these kits are designed especially to be used in all conditions.
Remember it doesn't matter whether you want cheap and cheerful or fine art quality - the images should always be individual to you, a work of art, an investment, something you can look back on in years to come.
There are so many photographers out there all offering different things, so choose the one that's right for you, the one that has the passion, that takes the time, has the patience, the eye for detail and you won't be disappointed.