Hi TAA IT,
You haven't said what type of files you work with. If it's just office, photoshop etc then you don't need XSAN which is designed to be used by Final Cut.
Apple did have a Promise Vtrak (E610FD) available for a while but no longer offer it for sale. You can pick them up second hand. The difference between the still available channel model and the Apple one is the firmware and the support offered by Promise. The channel version will work with an Xserve no problem at all but if you want the support and better performance from Xsan then the newer X30 or Apple Vtrak would be the way to go. One other annoying difference between the channel and apple version is the lack of support for drives larger than 2TB on the Apple version. There isn't a newer firmware to allow them to be used and probably wont be.
The Xserve is still a great machine and with hot swapable parts, redundant PSU's and Lights Out Management not to mention the space saving against a mac pro it's still a good machine to use as a mac server. There should hopefully be a new Mac Pro available sometime this year which might change things and even the current Mac Mini server is an option if you invest in one of Sonnet's expansion chassis which allow PCI-E cards to be used with a Thunderbolt adaptor.
Only thing on your list is the backup? How are you going to be moving the backups offsite? If you want to be rid of tapes an online backup might be quite expensive if you have a reasonable amount of data. You could check out iosafe.com who make very nice fire/waterproof disks which you should be able to store offsite easily enough. Something like the excellent Carbon Copy Cloner could be used to automate this process.
Hope that helps