The Mac mini has a disc reader/player, and reads all formats of standard DVD/CD/CDR etc. It doesn't read the HD standard discs, nor does it read SACD.
I don't think you'd hear a difference between a BeoSound 4 and a BeoSound 2300 player -- both would convey the musical signal to your active speakers through Powerlink, and it's the quality of the speakers that is decisive as far as the quality of playback is concerned. You may want to consider getting a good Digital-to-Analog converter and connecting that between your Mac mini and the BeoSound. The DAC inside the mini is not of audiophile quality.
How many CDs/DVDs you can fit on the server depends upon how you store them. You can use a RAID configuration, which means that if one of the harddisks in the server (mine has four) fails, all the information you have stored is still on the remaining three -- in other words, you have reserved capacity to ensure this kind of fail-safe in order to prevent loss of data. You can also just store your data raw and chance it. I'm using X-RAID on an Infrant NAS Server. Each DVD takes up to 7GB space; the CDs depends upon the bitrate at which you store them, I'd counsel 320 kbps as a minimum.
The Touch is a universal unit, it communicates via wireless (you'll need an Airport Extreme Base Station) and you only need one Touch. You'll find that the other members of your family will want to use it, though. The server I'm using is capable of sending several movies and different tracks of music out to various outlets in my apartment at the same time (Tvs, computers, stereos), and my daughter has demanded her own Touch.
You only need one, though, to get the whole thing working, and then you can just bring it along with you to whatever room you'll be using it in. It will communicate with your Mac mini via wireless.
I'm personally waiting for iRed 2.0 which will make it possible for my Touch to also transmit Beo4 commands to the hardware. At present I'm using Beo4 to switch units off and on.