I'm looking at the Deco P9 as I'm planning on building an office/workshop at the bottom of the garden that will utilise an existing mains supply but has no Cat5 and is beyond the reach of the house...

Thanks for the comprehensive answer and spotting my typo - I did indeed mean P9 and M9.

I'm trying to help improve a friend's messy home network. His house has dense, solid walls and floors, and no internal cabling other than mains. He is looking to improve both the internal wi-fi...