first question is why are you buying? The land will cost what your neighbour wants to sell it to you for. If you need the land and he knows he may charge loads. I’ve seen a neighbour charge £1000 just for a right of access over a drive when that right probably existed anyway.
dependent on the circs you will need a surveyor to draw plans for the land registry you will need a solicitor to register the plot and do the conveyance and your neighbour may use a solicitor and want you to pay.
If you are too narrow minded to pay for the solicitors overpriced time you may end up building an extension against the windows of which the neighbour or his successors in title build a 6 foot wall blocking your light, or refuse you access for maintenance. You could end up with an access narrower than your car .Or with a parcel of land you cant even access without trespassing.