Again, so sorry for the delay in responding. I have no direct evidence that Barrie bought Stonehill Farm for Michael, but Nico told that he'd bought Michael a place of his own in Sussex. This is confirmed by Denis Mackail on p550 of his 1941 biography. Referring to the early summer of 1920, Mackail writes:
There was a plan also just now which took up a good deal of thought and time. He [Barrie] had decided, on an impulse, to buy a cottage in Sussex, mainly if not entirely for the use of Michael and his friends. They were to spend their vacations there, with Michael as host, and what was to happen to it during the rest of the year hadn't quite been decided. It was a luxurious and generous idea, and he knew perfectly well what joy the same kind of possession would once have given the tenant of those houseboats [i.e. himself as a young man]; yet somehow, almost as soon as he had taken the first steps, he wanted to draw back. Perhaps Michael didn't jump at it eagerly enough, and there can be no doubt that solicitors and house agents were most exasperating correspondents. But in any case, now that a cheque for £4,000 been written, the cottage suddenly became "that confounded cottage," and the next stage was a rather awkward and difficult retreat. Moral: Beware of impulses. But he couldn't, even at 60. He had seen a toy, as it were, in a shop window, and had rushed in and bought it. Then he changed his mind, and had to pay again to get it taken back. He wanted to forget about it now. But if he saw another toy – and they were so seldom, if ever, for himself – he would do exactly the same sort of thing again."
So there you have it - a "cottage" rather than a farm, but it surely must be the same place.