i'm nto sure we have a lot of mac users, but i'm sure SOMEONE will be able to help you out.
in regards to reliability of dvd as a storage medium, basically nothing is 100% reliable. if this data is very important to you I'd probably make 2 dvd backup copies and also store it on an extra hard drive.
alsways use top quality blank media (this is the #1 cause of disc deterioration), keep the drive's firmware up to date, and burn at a reasonable speed, and your discs should be reliable. I'm hesitant to say 100% though.
if you buy a drive that has quality scanning capabilities you can scan your important discs every few months to check for deterioration and this may allow you catch a problem and back up the data again before it's too late.
unfortunately, i can't give any recommendations in regards to os9 compatibilities nor do i have experiences with pioneer burners.
if the scanning option sounded like something you'd like to be able to do, you should look for BenQ, Lite On, and or Plextor drives. (NECs also support scanning, but from what I understand are less reliable scanners than the other 3)
if you choose to stick with the pioneer, still keep in mind good media, up to date firmware, and burn at the rated speed and you shouldn't have any problems