Extend wireless network with Airport Express or Extreme?
TimeMachine isn't supported and doesn't like to work with drives connected to non-TimeCapsule or non-Servers. 
The express USB only works for printers, not drives. (no file server included)
Turning off WiFi on the ISP router and letting Airport run either in bridge mode (letting the ISP router handle everything) is the easiest option. I prefer to have my ISP router be a bridge and let the AirPort run the network to take advantage of it's software and logging / graphing tools. Some ISP don't like that and you need them to modify their upstream device - but most just work fine.


The benefits I see on AirPort are having one strong signal (turning off the ISP WiFi) and less interference as well as simplicity of managing the AirPort software.