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That's a frustrating situation, because no one of the scenarios stands out as far more likely than the others. I'd try cleaning the DVD first. Next I'd probably try other bootable DVDs, or burn another copy of the OS media to try. You can contact Apple, and with the serial number they will allow you to buy new original install media for something like $20.
I don't know if you have the ability to do this, but I'd tackle it by getting an external USB2 enclosure on eBay for $10, and then mounting a bootable 2.5" SATA drive (from a MacBook, if you have one), connecting it to the Pro, powering on it option mode (hold down "option" while powering on) and booting from that hard drive. If that works, you'll be able to verify the Pro is in good shape and test all its functionality, including the optical drive. If the optical drive works from within a booted-up system, then that tends to point toward bad install media, or the wrong install media.
And from within a system that is booted up, you can bypass the optical situation entirely by restoring a Time Machine or Super Duper backup to the internal HD of the Pro (which I'm assuming you'd have access to). You can even make a Super Duper image of the external hard drive, and then turn around and dump it on the internal hard drive, and then boot from that.
Anyway, let me know if any of this sounds like it could work or not, and we can go from there.