Sounds like a bit of a mess trying to deploy lots of different components to different architectures. Debian sounds pretty good. I like Ubuntu because I run it on both the server and desktop, so it's easy to have the same environment everywhere.
Ubuntu also has a more predictable release schedule than most distributions - especially Debian. I have yet to try the server version. How does it compare to Debian? Unfortunately, Ubuntu server doesn't support PowerPC, which the OP says is a requirement.
In my experience, Ubuntu server has been very similar to my Debian server work (I love apt package management). I'm currently using Dapper on the server since it's going to be maintained for a very long time.
Ubuntu has a more predictable release schedule, but I wouldn't recommend pushing the limit on upgrading your server. Unless you've got a lot of spare time you'll want to use a fairly mainstream distro. When a nasty problem comes up (and it will, trust me), you want access to lots of forums that show solutions to related issues.
Some general advice (not really directed towards the parent):
Just because you know how to run something like Gentoo doesn't mean you necessarily should in a production setting. Once you have a successful company you can worry about the benefits of compiling your filesystem from scratch. Pick an OS you're familiar with and run with it -- anything else is most likely premature optimization.
