I'm afraid that getting the material off would require a lot of effort, and some luck. It isn't an easy, straightforward process.
For example, look through this thread over at videohelp.com: http://forum.videohelp.com/threads/304047-Recover-not-finalized-disk
And this one: http://forum.videohelp.com/threads/310357-Unplayable-DVD-because-it-wasn-t-finalized-properly
The pro version of ISOBuster can probably do a better job of recovery for you, but since you aren't familiar with this type of problem, and are not comfortable with technical issues with optical media, I would hate to advise you to buy it, only to be unable to use it successfully.
Fixing the dvd recorder would be another option. Or finding the same model on ebay and finalizing the disks with it.