CD-RW or CD-R?

[AdamS]

I have a modern computer with plenty of hard drive space, but due to the volatility of my work, I'm constantly having to reformat. I'm looking into either a CD-R or CD-RW drive. First, it has to be cheap--$350 or less. Second, it has to be fast--less than 30 minutes a disk, and able to be run (possibly?) on a 75mhz Pentium. Lastly, (the clencher) it has to be external. Unless, of course, it's improbable that I'll ever get it to work on that P I pig. If anyone thinks they can help, it'd be greatly appreaciated.
If all else fails, I just want an external writer I can leave writing backup files and music CD while I go to school.
Thanks,
Adam