1. You can only have a maximum of 1 extra box. You can take an extra box subscription if you want it, although it depends on what you want to be able to watch on the other TVs as to whether you need it - apps are available for BT Sport, BT TV, and Now TV for many different smart TVs, dongles, and games consoles.
2. You need 44Mbps or above to get 4k Sport on the BT TV set top box.
3. Free to air channels are only available via a TV aerial.