A 1.8 backup zip might be too 'removed' for the current version to successfully restore. Might stand a better chance at restoring to a 1.9 'sandbox' ... which I'll be glad to offer should you like to try that first.
http://moodle.tcea.org/sandbox/
Contact me via IM in this system if you go that route. I'll have to set you up with appropriate levels to 'get 'er done'! ;)
Once restored to the 1.9, then, make a 1.9 backup and try the restore to the 2.3.
To avoid legacy files in the 2.3, create a minimal course. What one calls it really doesn't matter. Minimal means no options chosen during course set up cept those required. When you get to the point in the process of assigning users, instead, go to the Course admin menu and restore. Use the 1.9.x backup zip file to restore, but the important option is to 'restore to this course' (the one you are currently in).
Moodle will first attempt to convert the course to a 2 backup (.mbz) and use that to restore. One can find what Moodle does in /moodledata/temp/backup/ - even if it fails (might be able to re-cover at least files that were contained in the 1.8 course).
'spirit of sharing', Ken