Capital allowances for buildings are complicated, and you really need to get expert advice. I am not an accountant, but mine told me that repairs to existing buildings could be claimed as a business expense, whereas new ones couldn't. Hubby is right that if this were a new build house, you could reclaim the VAT. However, I'm really not sure about a new build stable.
As Dan says, for this to be a business, you have to be doing it with a view to making a profit. Obviously with your startup costs, you're going to make a loss to begin with, but you need to do some projections to see if you are going to be profitable after so many years. You don't say what you plan to do with your land to bring in income, but a good first step would be to do a business plan and see when (if!) you might start to make a profit, and go from there.
I read some tax case law a while back about an equestrian business, and the test they applied was something like "if this business continued to run in exactly the same way, would it ever be profitable". When the answer was clearly 'no', they were disallowed all tax breaks.
After that, I'd recommend you find a good local accountant (preferably a farm specialist), and have a chat with them. They'll be able to advise on the legalities and will be able to set up the business side of things, register you for VAT etc.
HTH!