Always get more than you think you actually need. It saves on having to do it over a 2nd time.
Make sure you get a receiver that has all the bells & whistles you want it to have & make sure it has preouts for adding separate additional amplification later on down the line. (Sounds like you will need it for such a large room.)
I'm partial to Polk speakers since they comprise the majority of the speakers in my setup. My oldest pair are my mains closing in on 20 yrs of service. So if I was going for inwalls, I would look there first.
Good luck.