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Wall mount TV; wanting AV flush box w/ power

Hi peeps! I'm looking to wall mount my TV (55"). I can likely managed the TV itself, but I'd love to have an AV box in the wall to hide the cabling and to provide power. Electrical and data services can run from the roof space into the wall. Wall itself is uninsulated (middle of the house), and I'm not aware of anything else in said walls. I'm likely to DIY most of it - particularly the data side - but I do need a licensed electrician who can connect the power and provide the paper work behind it. I'm open to suggestions on how big a box to install under the wall mount as well as any other ideas to future proof the installation.