I don't know if it will help you, as you and your user's needs may be quite different to mine, but this is the set-up I use for a small company that I do part-time IT stuff for. It works pretty well for them so it might work pretty well for your lot too:
This is for a small co. of about 12-15 engineers, (depending on how many part-timers your count). They do lots of computational modelling, so lots of storage space and CPU is needed. This sounds like it might be a bit similar to your needs (if you're going to do anything interesting with that large collection of videos & multimedia)
They have one "main" file server for project work. It's a white box PC from the local shop and has a couple of TB of hard drives in it. It runs ubuntu server with samba (like all the other servers in this co.). It has needed work about twice in the last 4 years.
They have a couple of old Dell workstations which are too slow to do engineering on now. One runs an external-facing FTP server (it could probably run a small website if needed, too) and one runs an internal wiki and a few other similar tools. I could probably move some of the internal stuff into the main file server, but we had the old machines kicking around, and it's useful to be able to fix stuff without breaking the whole network for the whole company at the same time.
We have a modelling file server, which is a big Supermicro rack server. It's a 4 or 5U box, because they have an open-plan office and nowhere to put the rack, so the rack-mount servers have to be very big (for what they are), so they can be quiet. This has space for 8 hard drives so you can pack it out with largish drives and there will be enough space for all but the most data-hungry organisations. It's expensive compared to the white-box PC, but if you really need the extra space, it can be difficult to find an off-the-shelf machine with space for more than 6 hard drives (and it's a lot easier to replace one if one fails, too).
We have a backup file server. This uses rsync to mirror the (newer) contents of the other two servers, so that if one of them falls over, we don't have a bunch of engineers sitting around while I get the train into the office, work out what's wrong, get the right part, fix it, etc. It also compresses the important (non-replacable) data every week so that someone can copy it to an external drive and take it off-site. Much cheaper than the internet connection that we would need to mirror a week's work in reasonable time over the internet.
All of the computers are cheap white-boxes from the local shop running windows XP or 7 with various versions of MS Office (whatever was current when the machine was bought). No-one seems to have any problem with the fact that the boss uses XP and office 97 while the new guy uses Win 7 and office 2010, and I have better things to do than make an issue out of it. We keep track of whose license is who's on the wiki. Most machines also have OpenOffice, but there is general user resistance to that concept.
We have a couple of PCs for doing number crunching. They sit in the corner and run VNC servers. If people need to crunch numbers they use them, otherwise they use their own cheap workstations.
In summary: buy off-the-shelf PCs for the users. So long as they have windows, office, anti-virus, etc. they'll get on with what they need by themselves. The hassle of getting people to use linux or OpenOffice is not worth the 250 pounds we pay to MS per computer. An off-the-shelf workstation or server with some extra HDs and some version of linux makes a perfectly adequate file server. Use sneakernet for backups.
As I say, your situation may be completely different, but I hope mine might give you some ideas.