Hey........I have a TX81z which uses the same Yamaha technology.......and that old module still makes sounds unlike any others.
I'll never part with it. It does take some time, but it isn't really that hard to learn about Yamaha's FM synthesis with all that algorithm stuff. I have some aftermarket manuals by Alexander Publishing that go into laymans terms explaining it all. You can always just start tweaking any of the settings..........save what you like in a user bank......I don't think you can overwrite the factory voices, but if you can there is a way to restore Yamaha equipment from that era to factory settings. Either by a simple button configuration reset or at worst reloading sysex. Don't be afraid to experiment. The DX7 will always be an unforgettable piece of synth history!
P.S. If you do buy one make sure a manual is included!