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I really wouldn't worry. most barns have a fibre cement roofing some with, some without asbestos. The type of fibre used in the roofing sheets is the less worrisome of the type of strands from my understanding, so unless you are going to crawl over them and fall through it would not stop me buying!
I would be more interested in finding out if the farmer is looking to sell up and convert the barns into houses...
We have found a house we like, with the one concern that is it next to barns that appear to be built with asbestos materials. From looking at them externally, I would think both the roof and side boards of the barns contain asbestos. House circled in blue, and barns circled in red in the attached.
I have no worries about being next to active farm buildings, other than the possible long term health risks of possible asbestos exposure. I dont really know the extent of the risk though. Nothing to worry about, or run a mile?