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Id not suggest renting it off the seller. If something happens before completion and either you can't buy it or they can't sell it, then it will be very awkward if you're living there. Maybe even more awkward given the personal connection.
I doubt your and their solicitor would knowingly let it happen, so you'd have to keep it a secret from them too.
If there's a couple of weeks' gap maybe stick your stuff in storage and Partridge it. It could be fun.
He is a really good friend. Best man at my wedding. I trust him about as much as you can trust somebody.
However, it is his company's property strictly speaking so everything would have to be formal anyway.