I have a Linux based file server with a RAID 5 array - Amongst other things, all my desktops back-up to it daily via rsync / cron.
You need a minimum of 3 disks for RAID 5, and they should all be the same capacity (otherwise all of the others will only hold as much as the smallest disk).
Replacing a failed disk is as simple as dropping it from the array, power down, swap disks, power up, add the new disk to the array and it should automatically start rebuilding it. If you have hot-swapping, you can skip the power cycling, or you can configure a spare disk in the array to automatically add and rebuild itself if an active disk fails.
No idea with regards to appliance like units, I haven't seen any where you can install your own OS on. Those things generally use proprietary RAID set-ups (so if it fails, you won't be able to recover your data from a PC), they're comparatively expensive and they're mostly limited to a maximum of 4 disks unless you get into stupid amounts of money.
If all you're going to be using it for is serving files, you'd probably get away with a low powered Intel Atom based set-up and a bunch of disks.