I appreciate it's not quite the same thing but I've bought land off the Parish Council before.
I think the first thing to establish is whether the land is being used in any way - including informal access/ recreation / amenity by the local community. If it is currently being used it is unlikely you will get anywhere with an application to purchase, unless you guarantee this local community use will be continued. This would have to be acheived by a covenanted entry being added to the Land Registry document.
The second thing is probably to demonstrate your intentions for the land. I would suggest that you offer to include a further covenant, again to be entered on the Land Registry Document, that there will be no development of the land. This would preclude buildings and commercial use other than that which falls within the definition of forestry. Typically in a belt and braces approach to protect against land developers profiting from such transactions a further clause is added (this time just to the contract of sale) such that if the value of the land rises as a result of development the council gets 50% or more of such a rise in value.
The thirdly, and the one you have to be more careful about, is how the council may seek to protect adjacent properties from any "nuisance" arising out of activities you may carry out. You should offer to discuss this with them, but I would caution against signing up to a blanket clause of "any" nuisance - it would just take one neighbour to fall out with you and you would have constant problems. (nuisance is a difficult thing to define and is very subjective - how much of a nuisance does a nuisance have to be before it is a "nuisance"!)
Finally you might offer to include a clause to the effect of giving the council first refusal should you decide to offer the land for sale in the future.
However if you really have no wish to develop the land then typically the council will be more open to the idea of you 'buying' the land "leasehold" from them rather than the freehold. This way the council realises some of the capital value of the land, frees itself from the costs of many liabilities (injuries, issues with adjacent properties etc.) but they still retain control in terms of preventing development and keeping neighbouring owners happy. This is a good option for the council as it still leaves the door open for them to benefit from potential development in the future - it's about them being able to demonstrate good value for the taxpayer from their decisions, with a leasehold arrangement they haven't ruled any benefit to the tax-payer out in the future.
I would suggest that a 25 year leasehold at a rent of £10 per acre (this would typically be payable in advance as a lump sum to reduce admin. costs would be worth about 35% of the value of the land freehold. This assumes that there is no obvious development potential.
You will also need to think about how neighbouring property owners may react - if the council is interested in selling to you it will consult with them as stakeholders. I think you have to play the card that they will benefit from the land being protected from development. Others may feel depending on the type of residential property you have that the value of your house plus the land together will be greater than the sum of the two seperately - this is debatable at best however. Again depending on the nature of the adjacent properties others may feel this is a good idea and wish they had thought of it - it isn't impossible that a bidding war may begin.
Whatever you do don't mention pigs! Whatever people say about how clean they are - and yes they are - but Pigs do smell. Even if they didn't (but they DO!) the perception is that they do and you need to keep your neighbours onside. My experience with pigs (Iron-age (tamworth x wildboar)) is that 2 pigs per acre will decimate the ground flora and damage the bark of trees in a single season. Pigs were never kept in woodland - they were herded through woodland during the autumn.
Finally find out who your local councillors are. Look at their homepages on the web. Look at their interests and see if there is one (or more) who might share your interest in conservation, or some other aspect to what you intend to acheive. Perhaps you might be interested in offering a couple of open days to local schools. Just try and find an angle that might interest one of them in the posibility of what you are intending to do beyond just a financial transaction. Then contact that person first and get them interested in this project - even if they only see it as a photo-opportuntiy for the local paper!
Good Luck!!