Seems a bit strange that the owner would arrange to meet a potential purchaser on a day that bars access to most of the wood.
My first concern would be whether or not you will have access problems on a number of shooting days or even throughout the shooting season.
Before worrying about that though - where and who are you going to sell this timber to? You say it is only good for firewood and that you propose to move it 600m to a loading area accessible by artic, I guess then that you propose to sell it as cordwood? Do you have machinery to move and stack the timber at your lorry loading area?
Essentially you need to find a price per tonne or per m3 which you can sell this timber at, estimate the volume or tonnage you have and work back from that, allow:-
haulage cost to your end user or agree a price at roadside.
cost of extraction to your loading area
assuming you will fell, sned and process yourself - how much are you going to cost a days work at and how many days do you estimate to do the job, don't forget to allow for a tank of petrol and oil per day
cost of travel to and from the wood
If you cannot estimate the volume of timber in the wood then I would try to work on an outturn basis - you pay a rate per tonne or cube for the timber you take away
see if there is anything left to pay the owner with.
I would look carefully at the windblow and make sure that you can work it safely and that it is actually worth bothering with.
Also check that there is a felling licence in place and clarify who is responsible for ground preparation and restocking, does it actually allow the timescale that you indicate?
Step carefully and think twice as carefully before you take it on and don't even think about making any agreements until you have been right through the wood and have a full understanding of any restrictions on access.
Cheers
mac