It's worth noting that if it's only doing file and media serving, and you have less than ten users, the non-server version of OSX will work just fine. The Server version adds a lot of extra services, and streamlines some of the management tasks, but it's not required.
I'm doing something similar to what you are; I have a non-server Mini in my living room hooked up to an external (FW800) storage volume that's accessed by all the other computers in the house. It is also a media center; it's connected to the TV and home theater system, and runs iTunes as the central iTunes library and iPod synch point.
With the TV on, I can use it to play streaming or downloaded videos. With the sound system on, I can control it via the remote app on an iPod Touch to play music, and also access the library through iTunes on any Mac in the house. Even with both off, it acts as a fileserver for other computers. And, if I need to run something remotely, I can use a remote connection to display its desktop from another computer even when it's running headless. It also has two additional shared HD partitions that each store a Time Machine archive for my two Macs, which back up via the network.
Because it's a full computer, I can run Bittorrent or similar apps on it without leaving a main computer on. Another advantage is that I have a USB external backup drive, which the server does automatic backups and versioning to with a 3rd party app (so I have more control than Time Machine allows), which means I don't have to think about backups for any of the data on the system. This is better than RAID for my use, since in addition to basic disk failure protection, it also protects against accidental deletion or file corruption (or accidental editing), at the cost of possibly losing a few hours' work in the event of a sudden, catastrophic disk failure. The tradeoff is well worth it to me (and the hardware is cheaper!); RAID1 is pretty much only for people who need zero downtime (like on our headless Mini Server at work), and RAID0 only for people who truly need massive data throughput (video capture) and idiots who don't care about their data.
Had the Server version of the Mini been available at the time I bought mine, I would have gotten that, but it's not necessary, and even if I did have a server Mini, I probably wouldn't use the internal drives for data storage, since 1TB isn't enough for my needs (we don't at work; we use a hot-swappable WiebeTech RAID1 array that can do on-the-fly rebuilds).
My Mini Server has an optical drive, but my laptop doesn't; I can either hook up a cheap (~$50) USB2 external optical drive when I need to install software (I have one for BDs and DVD ripping), or use a remote disc from another Mac, which works fine.
Given what you're describing, it sounds to me like you want a very similar setup, and that it will probably do what you need it to. Were I you, I'd get a Server mini and hook up the external drive unit you mentioned to it,
