Airport Express and AirPlay I'd like to set up an AirPort express connected to a HiFi as an airplay receiver, acting as a device on a wireless network.

At some point in the future, I'd like to extend my wired network to the location of the airport, place the airport on the wired network, continue to use the airport as an airplay device but also use it as an access point for the wireless network.

Will both of these situations work as described?
sure, this will work. when you extend your wired network, change your airport express to either extend your existing wireless network or setup it up as an access point.