| I think it is! Certainly it meets your requirements: No internet access for the datacentre machines. Control of what goes on. Ability for testing. Ability to add your own site specific packages.
Iwould use three boxes. The only one connected to the internet would be the install server. Also in your test environment would be a pure test machine, setup exactly like the data centre. The third would of course be in the data centre.
I would defintely use Stable. Which for a while yet looks like potato. If there are newer versions/packages you NEED then I would use the net connected box to compile potato versions of woody packages. ie, My net connected box would ONLY have stable sources for binaries, but would include testing/unstable sources for source.
Now I have only done this at home, but I am helping push Linux at work, and I am trying to use this feature (as well as many others, of course!) to recommend Debian.