Depending on the amount of data this is probably possible within your budget. The drawback with NAS is that you need to learn everything you need to know with direct attached storage, and then stack on top of that network knowledge. So NAS is inherently more complicated to research, plan, and configure. It shouldn't be more complicated to maintain, but will be different.
You could check into running one of the open source NAS options I mentioned on an Atom CPU and save some money. Otherwise the ballpark of what you're looking at hardware wise is something like this
microserver. All of the open source options I mentioned use a file system called ZFS. It is inherently resilient, checksums all data, does not suffer from silent data corruption, bit rot, or the RAID 5 write hole problem. It can create periodic (scheduled) snapshots which in turn can be exported to an external disk. Those exports contain verifiably identical data to what's on the NAS.
I haven't tried it, but hypothetically you could upload those snapshotted exports to CrashPlan. That portion really depends on your available upload bandwidth and how much changing data you have.
FreeNAS vs NAS4|free (I put the pipe in because this message board must think it's spam if I don't) are similar but have different maintainers. I *think* FreeNAS is based on FreeBSD 8, ZFS v15, with a newer interface; and NAS4|free is based on FreeBSD9, ZFS v28, with an older interface. I have used FreeNAS in VM, it's pretty straightforward, I haven't gotten around to evaluating NAS4|free but I like the idea of using a newer (current) version of ZFS. Key is both have built-in support for AFP using NetaTalk and support Time Machine backups (to the NAS as if it were a Time Capsule or Server).
Nexentastor I prefer overall performance and interface wise, but it lacks built-in AFP. You either have to install it yourself, or figure out how to do NFS which might be like a really simple calculus problem. i.e. if you know how to do it, it's cake. If you don't, it's annoying research.
AFP I get roughly 35-45 MB/s throughput with gigabit ethernet. If I use NFS with the async option, and this mount command in Mac OS X, I consistently get 100+MB/s on sequential files (like AAC files, movies, Raw images; piddly text files are much slower performing):
Code:
sudo mount_nfs -o resvport,rsize=32768,wsize=32768 192.168.1.137:/home/chris/Music /Volumes/NFS
Naturally the IP, username and mount point may be different for you. To set this as a persistent automount option, you can use Disk Utility's NFS mounts option, and break up the above command.
Anyway, the point is, it can be done, but it's a layer or two more involved than just locally attached storage. If you can learn new things, tolerate getting frustrated, and commit to making it work, then it's a good route. If you want something more plug and play experience wise, a NAS is a little rough around the edges.
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Oh and the hardware, Macbook Pro 8,2 and Macbook Pro 4,1, and a Linksys WRT600N running dd-wrt firmware. With non-pristine CAT 5e cables. Really, I probably shouldn't be getting 100MB/s with the cables I have. Just make really sure you don't kink them, have furniture or staples putting pressure on them, or making them to tight bends like 90 degrees around a corner, or near power lines or worse fluorescent ballasts. Ethernet cables are a bit pissy.
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Oh and NAS software. I'm biased against proprietary software. I don't want to get stuck in warranty let alone out of warranty, having to replace or buy proprietary hardware to re-gain access to my data if some bit of hardware fails. If you buy an Atom server that holds two disks and you outgrow it in 2 years, you can put the drives into some totally different hardware that holds 4 or 6 or more drives, and still have access to your data. I'm sure service/support for proprietary solutions are competent and helpful, and all that - so open storage is my bias, which is why I've suggested the three NAS options I have. All are managed by web browser interfaces, just like proprietary NAS's do.
FWIW, Nexentastor scales to
petabytes of storage and is used for extremely large enterprise class storage without hardware RAID controllers. But is free for storage arrays 18TB or less. So it's not like it's Tonka Toy stuff, just because it's free. And FreeNAS and NAS4|free are, again, quite similar and can easily manage tens to hundreds of TBs of storage without a problem.
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http://www.nas4 free.org/index.php/nas4 free-home/
remove the space after the two 4's.
http://www.freenas.org/
http://nexentastor.org/