When I was still actively working for clients I had a similar situation. I don't remember exactly what versions of everything, but I do know that the hardest part, and reason I didn't do the upgrade, was postgres upgrades. There are many gotchas.
You are in for a painstaking task, unless someone else knows something I didn't.
That's interesting. I'll do some experimenting first but maybe an option would be to use a new Linux (so as to get all the latest packages/tools/security updates) but to build an old AOLServer/PostgreSQL from source, so as to benefit from the new platform without breaking too much of the application.