How are you backing up the client machines? Are you using software or scripts or rsync or ... ? Is there value in having image backups of the client computers as well as the data? It sounds like you're in need of a backup server/storage depot-ish solution. A NAS would be the simplest way to get a chunk of network accessible storage, but may not offer much flexibility or expand-ability in the future.
Here at the office we have a Dell server for holding primary data (with nightly backup, RAID drives, on a UPS and has ECC ram - i guess that checks all JBI's boxes nicely) that backs up using an external hard drive that I rotate offsite. However, that is the last place I would go to for a backup and only in the case that my other options are exhausted because my other options are (1) better and (2) quicker. A few years ago I bought an Acer Home server that uses an Atom processor (for low power usage, looks like this http://www.newegg.com/Product/Product.a ... 6859321013
) with 4 drive bays and WHS installed. Despite all the computers being on a domain, I had no problem installing the WHS connector on them and backing everything up from every client nightly. I was also able to backup the server with no drama and have used the restore functionality multiple times with success. I also use Crashplan on the server and the backup server (as a target) as well as subscribing to Crashplan central for data backup hourly.
So, to recap, data is backed up nightly to external drive (includes server drive image for restore), nightly with WHS for all data and drive images for all machines, and Crashplan hourly and nightly for all data backed up locally and to the cloud.
While the acer machine I use no longer appears to be available, this HP http://www.newegg.com/Product/Product.a ... 6859107052
seems like a good alternative and WHS2011 http://www.newegg.com/Product/Product.a ... 6832416443
, while not possessing Drive extender, can easily backup up a bunch of machines to a no-longer-astronomically-expensive 1TB or larger drive. http://www.newegg.com/Product/Product.a ... 6822152245
Any way you go, make sure your backup is robust and as easy to manage as possible, because if it's not, you won't want to do it (I used to hate doing the tape backups at my last job) and there's nothing worse than a backup scheme that doesn't actually protect your important data