The basic mini should have no problem playing mp3's or DVD's. It's much more powerful than some of the G3 macs that can do this with no problem.
My 80 gig Mac drive shows 74.52 gig available after formatting. The "System" disk is 1.37 gigs. The "Library" is another 1.2 gigs and I'd round it up to 5 gigs after software and installations. Leaves about 69 megs for your stuff, including your apps. I've got 12 gigs of apps installed and I've got way more apps than I would ever put on a CarPC.
Ripping DVD's - try "Jack The Ripper"