Macs autosleep? They have energy savings settings like Windows and those can be changed to and idle time or be turned off.
I have an old Power Mac G5 tower, which is the same form factor as a Mac Pro, and my parents use it to check e-mail and it serves as a file server for the whole house, as well as a print server, and it used to be a 24/7 torrent box as well. While the hardware inside the G5 is very old, after it is retired I plan to use the case and fans and just put new guts inside.
The hardware:
The form factor is awesome. It's hard to beat those looks and the airflow. That being said, it has this design because the hardware is intended to work really hard and needs that much cooling. File serving, sabnzdb, Apache, even a MySQL server, all are very low demands on a computer. It will work great, but it will draw a lot of power even if it's not doing a lot (any desktop workstation will).
If that's not an issue, and you have a good price on some older units, there's no particular reason not to use them. They're huge, but neat cases.
There are 4 internal HDD bays inside the Mac Pro, but I've seen people sneak in an extra two inside the very large PCI bay, since most people don't use all 4 slots (they're all full length, too, so lots of room in there for a 3.5 inch drive). I think there is a total of 6 SATA connectors inside, 4 for the hard drive bays, and two for the optical bays (you can easily use the optical bays for drives as well, just as one would for a PC). You can always add another SATA card (make sure it's got Mac drivers) and even use SATA drives in an external enclosure if you run out of room, again, just as you would for a PC.
OS X and OS X Server:
You really don't need OS X Server to run a Mac as a server. The great thing is that Apple made the server portion of OS X dirt cheap (something like $30 extra on the $20 for the OS, IIRC) so if you do want to use some of those extra features (legally) you can without having to worry about losing a lot of money.
That being said, as someone who has bought and used Mac OS X 10.7 Server, I've never needed the extra stuff for my home server and/or media functions. At all. Some of the tools looked really neat, but they're intended to be used when you have a Mac acting as a server for several (think 50, 100, or more) Mac clients. A lot of things are geared towards running different kinds of server applications, like Calender servers and e-mail servers.
Mac OS X client includes almost all of the parts found in the Server edition anyways, like a full Apache web server, everything you'd need to run a file server with multiple protocols, remote log-in abilities, printer sharing, etc.
Server does come with additional GUI utilities to make some stuff easier. In theory it should make setting up NFS or webDAV easier, but personally I didn't find them very helpful in my small-scale network (5-6 clients on the network total, only 2 or 3 accessing the server regularly).
Like I said, though, it's pretty cheap, so it's hard to feel cheated. It certainly won't hurt anything to buy the server edition.
In general, Mac OS X (with or without server) is fairly comparable with Windows 7 as far as abilities go. Setting up proxies and Apache is fairly straight forward, depending on how complex you want to set things up. I've never needed to set up something like a VPN myself, but people tell me it's not hard to do on Mac OS X. It's very easy to set up whatever download manager you want to use, and Mac OS X has a fair set of XBMC-related applications (see
Supplemental tools/Mac OS X (wiki)). I won't say it's drastically easier than Windows 7, but I've always liked Mac OS X more and found it very flexible yet straight forward. (Comments about Macs being "locked down" are typically misguided and only apply to the iPhone/iPad's iOS, which is a totally different monster).
Things are very userfriendly to set up, and if you wish to dive deeper, you can break open the command line and do just about anything a linux OS can do. (technically speaking, Mac OS X is based on BSD and not Linux, but most of the major "command-line" applications available for both).
The only thing I find lacking about using Mac OS X as a media-based server is the lack of a good PVR/DVR/Live TV backend (something I've only recently gotten into). Other than that, it's a good choice.
If you go with just the normal OS X and not the server version, I would recommend sticking with Mac OS X 10.6. In 10.7 they took away some of the file sharing GUI settings, and SMB/SAMBA requires extra steps to work (server wise) with XBMC. 10.6 also seems to have less issues than 10.7 and 10.8 with Mac Pros (and in general). No real "show stoppers", but for a server I wouldn't worry about the extra features that 10.7 and 10.8 bring and would rather go for stability.
