There won't be extra wear and tear on a 10mm blaster because they are buiit heavier to begin with. However, YMMV on the user issues. The only reason to shoot 10mm is if you are shooting real 10mm ammo (Underwood ammo is the only ammo I use, 165gr. Gold Dots are my favorite). If you plan to use...
Have a link? Most BCM uppers I see for that price are minus charging handle and bolt carrier group, which is ~$100-200.
That being said, I have seen complete Spikes Tactical rifles in the last couple weeks for ~$700-800 which is superior to the M&P.
The A2 FSB is the most secure gas block you will ever find. Even if you want extended handguards, I would still get an A2 FSB and chop it to keep the gas block, then add your preferred backup sights. This is my favorite setup.