RAID (0, 1, 5, 6)
Here are a few resources for explaining RIAD. This helps understand how the servers manage all the SATA drives.
RAID 5 & RAID 6 - All You Need to Know as Fast As Possible
Much longer video Introduction to RAID
If you prefer to read through it A Beginner's Guide to Advanced Storage
SYNOLOGY DSM
For more info on Synology's operating system DSM (DiskStation Manager) and the types of apps you run on it, see this page:
http://www.synology.com/en-us/dsm/for_business#tab_2
NAS seems like it could do the trick, but can a NAS host email as well? if so can you suggest a few options/brands?
Synology's DSM does not come with email installed, but you can add Mail Server & Mail Station through the Package Center for free. It offers several other add-ons for websites like Wordpress, Cloud Station, Directory Server, etc. There are also media apps that manage pictures, music, video as well as security apps
is a pogoplug worth anything?
I'm not too familiary with this service, but it looks like a NAS will also offer you similar services. The Synology will and so will ReadyNAS from Netgear. I do not think these work with iPhoto and iTunes directly. Most NAS media servers provide their own interface (e.g., HTML) rather than allowing for iTunes or iPhoto to manage it.
It's true there is an iTunes server for Synology's DSM and Neatgear's ReadyNAS, but it's more like a network connected computer with an iTunes library you can access from another Mac with iTunes. Not a great solution for sharing media, unfortunately. Apple does not seem worried about improving this either. They force all media management through a single Mac computer that then shares it with other Macs & devices. iTunes & iPhoto are the gatekeepers of your media in the Mac ecosystem. (I can access the iTunes media from another computer, but have to navigate the file folders on the NAS. Those files can be copied to any computer. You could even link a second computer to that iTunes folder to see the content via iTunes, but any updates won't show on the second iTunes.)
I've chosen not to use DSM to manage my media. Their software and ReadyNAS's software is weak. I have an Apple ecosystem, and I use my NAS strictly for storage, not media management. So my music library is on a MacBook Pro that uses iTunes to catalog the files and I store those files on my NAS.
iPhoto is a different story. As I understand it (and I may be mistaken), you cannot simply copy your iPhoto database over to your NAS and relink it. It has something to do with how the disks are formatted from the beginning that limits iPhoto's database ability. Again, I have not tried this yet, so not too confident there is a solution out there.
Which NAS to buy?
I like my Synology DS1813+. It's a bit nicer and quieter than my ReadyNAS, I own. The DSM is comprehensive and gives you plenty of options for managing the system. It's also fairly intuitive, which helps. From the start you can manage your different disk partitions, users access, TimeMachine backups, schedules for shutdown and backup of the NAS itself, and the individual drives. You can add services for free like Video Station that is a web-based iTunes and has mobile apps to provide access, and Magento, an ecommerce platorm.
I'm sure there are solutions for you that are smaller and less expensive. Take a good look at your budget and how many years you want to get out of the NAS. Upgrading can be rather tedious and cumbersome to set up. Decide if RAID 5 is too much for your business or if it's crucial. If not RAID 5, then what's your backup strategy? I belive everyone needs a solid backup strategy whether its Dropbox or RAID 5 mirroring to another RAID 5. List out the MUSTs you need in a new server and make sure the one you get can handle it. Some of the smaller storage NASs don't offer email server capabilities.
Hope this helps. Let us know if you have further questions or need further clarification.