The interaction with the modem works perfectly. The whole flat thing depends a little on how big the flat is. After three walls, the signal becomes noticeably weaker. That's why I use two of these stations in a five-room flat - at opposite corners.
No, the TimeCapsule has a built-in HD drive. The AirPort Extreme does not.
You don't have to plug in a WiFi printer via USB. It works directly via WLAN