If your new system is powerful enough (and no, I haven't any experience
of this but I'm planning to do something like this myself :-)
Experiment with UML (User Mode Linux) and Gentoo. There's a good chance
that if you get a minimal gentoo system running, then tell it to install
the packages you need such as tetex 2.5, it will hopefully work fine.
The portage system *should* resolve the necessary dependencies.
As I say, I haven't done this. But it's an idea to play with ... and if
you can get UML running you can easily run an old setup as a program
inside Dapper :-)
Cheers,
Wol