The best thing to do would be to contact the estate agents selling it, and ask to have a look at it.
If it's equine registered land then you'll be ok to apply for planning for whatever you want - if it's registered as agricultural then there are several problems you may come across.
You can apply to have it re registered as equine, but that may be turned down and if so then you are only allowed to "graze" your horses there ie you cannot have any permanent structures or hard standing, or keep any tack or feed there, so you'd only be allowed mobile shelters, and would not be able to keep a trailer there.
With the land itself, walk around and find out what it's like all over, you can very often tell from what grows there whether the land is boggy or well draining, like horsetails will only grow in boggy conditions, and does there look as if there's any ragwort.
What are the fences / hedges like, and will they need reinforcing?
Is there any water supply - if not you may be able to have a bore hole sunk.
Any access problems?
We've been through all this ourselves, and we got to be very well versed in all the legal stuff as we had complaints about what we wanted from some NIMBY types who lived about half a mile away !!
Good luck with it all :o)