I work in IT, so as I would see it, the time capsule would not be the optimal solution in your situation.
The time capsule is more for the person who A) Has a mac and does not have a pre-existing network and wants an all-in-one solution with additional storage, wan/lan ability, and wifi A-G-N, or B) has a network but wants to add storage and wireless N to the mix, or C) loves all things Apple and wants one just because they don't want to use any other product.
The breaking point for me on a timecapsule is that its a single disk solution. Disks fail. Not the optimal thing for a Backup solution. I say this having very old 10 gb drives still running flawlessly in their 12 year-old housings, but still, its not what you'd want for a storage/archive solution. I'd rather get a NAS solution with Raid. I've been looking at some 2tb and 4tb NAS drives from LaCie which is likely what I'm going to go with.
The Mac-Mini server is interesting... but only because I've not ever played with an OS X server before. Having tons of unix/linux/bsd experience, other than the ability to put "OS X server experience" on my resume, I don't imagine that its so vastly different from using other BSD style OSes -- at least when you're diggin in the guts of the thing. The UI and the ability to manage Macs and Mac Users would be the main selling points - certainly not the storage capacity. If it had eSATA on the Mini... then that would be a different story. An eSATA connection would be as fast as the internal SATA bus, and open up a lot of relatively cheap eSATA external raid enclosures which would be nearly server-class in performance.
Theoretically, you could get a very cheap box and install a bsd/linux distro on it, put some drives, and download some of the free OSS to give it AFS compatibilty. Not exactly a time-capsule or OS X solution, but likely to work. You could probably find a distro that would work well with the Mini G4 as well, although that would be rarer and you'd have to use what was available on the Mini - which would likely be a USB external solution for additional storage space. Not the fastest thing to do, but certainly cheaper than any out of the box solution involving NAS Raid or getting an OSX server.
It all depends on your budget. I'd recommend either the NAS or a computer-server solution that has RAID though - Raid 5 is ok, and can tolerate the failure of one disk, but is a 3 disk solution. Raid 1 mirrors, and is good for 2 disk solutions. The thing to remember about raid is that when you do anything other than raid 0(striping), you lose the capacity of at least one of your drives. So if you have 2x 1tb drives, and do a RAID 1 config, you get 1tb total storage, as your data is being mirrored. If you have 3x 1tb and you use a raid 5 config, you get 2tb of storage. Nothing can help you though in either of those two configurations if you lose more than one disk to failure. And if it does fail, you have to replace the disk with one equal in capacity, preferably the same make/model as the other disks in the array. It also takes time to rebuild the disk from the array before the system is fully usable again - which is worth it because that means your data will be backed up once again. There are other raid configurations that provide multiple disk backups, hot spares and the like - but these all require a raid system that can handle four or more disks, and the price goes up the more disks you need. Another important thing to remember is that in RAID configs, all the disks must be identical in capacity otherwise it won't work.
Hope this helps,
PV.
