Now it's occured to me that as the object of having a gentoo install previously was so that I wasn't in the perennial distribution upgrade every 6 months, that I could just update/upgrade individual things over the months.

Well I can do that with this as well can't I? (well I'm presuming so)
I could be wrong, but the Knoppix HD install works but it leaves you with a Debian system that is split between testing/unstable trees. This split makes your system difficult to remain current in regards to updates and security fixes. This is not to say it can't be done. However it may not be trivial process.
There is a Knoppix derivative called Kanotix that is specifically designed to not have a split package system & allow easy HD installs.
Google up Kanotix to read more about it.

I hope this helps.
James