My starting point to any Mac OS 9 machine is Diehard Instant DAW.
From there you have a fully working cubase that will allow all those VST from Steinberg you mention once installed. Again, I would authorize all the key plugins.
Then I would install Logic 6 with all the updates and once it is working, I would copy the VST folder from cubase to another volume. After that I would copy that folder to the Logic VST folder.
After a good amount of reboots, you will diagnose what plugins refuse to work on Logic. Most Steinberg plugs need Nuendo or Cubase to work, but a few of them work also in Logic.
At the end you realize some plugs are meant for Cubase and others are open to any host.
Also some plugs work better on Cubase than on Logic.
Some more modern work better on Logic 6 than in Cubase.
Also, most Logic 6 own plugs are superior than stock Cubase ones.