| Hardware for that, to offer good uptime...
If the sites are going to be fairly low traffic then you are likely looking at 2+ harddrives in RAID-1 configuration, 1GB RAM (512 would suffice), and at least a 1Ghz processor.
Disk size is up to you, you can do what thousands of web hosting providers do and "oversell" your disk space, so for example:
Without overselling.. 600Mb per customer, for 100 customers means you MUST have 60Gb of disk space in total.
Overselling... Now imagine you know that on average your customers are going to use 300Mb of diskspace. That means you offer 600Mb, but you only need to purchase a 30Gb disk. This saves you money buy not having to buy large disks to support what your offering because you know your customers wont be using that space anyway.
If you want to create an offsite mirror, you can use software like rsync or write a custom script to do it.
Jason |