Airport Extreme vs. Time Capsule vs ?

I'm a new Mac user and I'm very interested in the Time Capsule / Time Machine solution to backups. A few questions I'm hoping to find info for before deciding how to proceed :

1 : Are Time Capsule and Airport essentially the same from a router standpoint? Is the range/performance good on both devices?

2: Does it makes sense to buy the Airport and add my own USB hard drive that can be upgraded at will?

3. Other than wanting to get totally geeked out with Apple gear, is there a reason to use the Apple networking/storage products over say a Linksys router with a Western Digital hard drive?

4. Could I attach a USB hard drive to Time Capsule, and use it as a network drive while TC handles the regular backups?

Thanks for any advice / opinions!

1.) Yes
2.) I did because I already had an AEBS and I believe backups are slightly faster with TC.
3.) User preference but IMO Apples works better and last longer.
4.) Yes but I would use a networking drive like a WD Livebook.

Time Capsule setup and working, effortlessly. WiFi signal strength is excellent and I've got about 4 hours left on my initial backup.
Like my friends have been telling me for years about Apple, "Stuff just works".