Both my current cars have head units that play MP3s off of data CD-Rs.

I'm going to want the same functionality in my 2001 'Y' C70 convertible T5 GT, once I've picked it up.

I know some cars have extra things built into their stereos, like alarm systems and stuff.

Can I fit a lovely flashy new head unit and not break anything? Are there any makes of facia adaptors and cable adaptors that people can recommend?

No, I don't own an iPod or anything like that.