Need AirPlay and a network drive - is having two devices my only option?
I believe that most of the features offered by the Express would be redundant with the AirPort Extreme, so perhaps the Apple TV would be a more appealing option. Since the Apple TV is exclusively a media device, you gain a remote control and video streaming.  The Apple TV will only output audio over HDMI or Optical, so you would need to ensure that your current sound system is compatible. The AirPort Express outputs audio over standard 3.5 mm as well as 1/8" TOSLINK optical.

An alternative would be to replace your current sound system's receiver with one that features AirPlay streaming: http://usa.denon.com/us/Airplayus/index.html
This would be significantly more expensive than an Apple TV or an Airport Express, but would cut down on the number of devices involved in your set up.