If you are looking for multi-OS solution, R1Soft is the only one coming to my mind, unless you are fine with setting up Cygwin Rsync. RAID 10 with 4 HDDs can certainly get you going for years without ever thinking of a need of backup restoration from a total destruction!!!
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Setting up a R1Soft server is not that hard, and the clients are very easy to install and manage.
Just to get an idea, I hosted about 12 agents on a C2D, 2 Gb RAM, 2 x 1.5 TB SATA and RAID 1. I'm not quite sure but I could bet the server could hold about 25 agents without any problem. If you get a quad core, 4 Gb RAM and preferably four 1.5 or 2 TB SATA in RAID 10 should be more than enough. But... it all depends on how much space you need and the interval you set the agents at.
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