Well there are a few things you should do. First and foremost, you should talk to the local planners. The sort of thing you need to know is 1) what is the position on infill development in villages in the local plan?
2) have there been any applications on that bit of land that have been turned down and why?
3) is there any other policy affecting that location (conservation area, flood plain, SSSI sort of thing)
Most planners are good about discussing this sort of thing and some of it (eg looking at local plan and application records) you can do without even talking to a planning officer (although you might need to pay a fee). I would talk to a planning officer regardless, though - they can give you much more detail about the scale of development that they might deem appropriate.
Other things to check out - highway authority situation and legal rights over access, utilities access (can you get drainage in and out?), legal title on the land allowing development (may be restrictive covenants).
Then you can begin to think about whether the development you want to do is the sort of thing that might be wanted by the market in your area and whether you can afford to do it.
As for valuing it, you might want to look both at the capital value that will be generated and the cash flow issues. Putting a value on it is fairly straightforward (once you have the planning officer's advice on what - if anything - might be allowed) but cashflow is a bit trickier as I guess you won't be a position to get quotes to help you budget until you have bought the site.
I guess my feeling would be do not buy the house simply to acquire the plot, because infill development is always a bit of a hot topic and my impression is that policy varies from planning authority to planning authority.
Hope that helps