It really depends on the type of horse you have, since some horses have a crazy long, thick coat, and others just get a thin coat (if you want to do full body clips). For small jobs you are probably going to be fine with smaller clippers, but if you have a horse that gets very thick and long I have these http://http://www.doversaddlery.com/...per/p/X1-1802/
They are not cordless, and they are not light, and they are also not really for clipping around the most sensitive areas, but they will actually clip (and clip, and clip, and clip - until my boy looks like he is made from marzipan). (I am not sure why they have some bad reviews in terms of breaking, since I have always taken care of mine while I clip, and after I clip, and mine never broke). One of my guys (the Fjord) has such a thick coat, that any other clipper will just give out!
If you have a horse with a short/medium coat, I think the A5 is a great clipper. I used to have a heavy duty Aesculap clipper which I loved, loved, loved, but left with a friend in Germany before moving to North America.
Blade size is a key too, as you're blade will have to be big enough to deal with thicker hair, our fine enough to do the detailed work!
Give us more info on what you want the clippers for (type of coat, etc)!! :)