I know you're looking for a CPanel backup server, but we ourselves have a Cpanel server that backs up to a basic server via Rsync. It's fast, efficient and a proven way to back things up.
We originally used an old server, with a cheap co-loc, to handle our backups. But then we found BQBackup and it completely works for us. No hassles. Good setup instructions. Zippy backups. Great support.
And for 100GB space at only $20 per month, it's a total time & cost saver. Cheaper than running a co-loc or dedicated (which, at the very least, will have to be managed).