Regarding CMS solutions, although I haven't really used Drupal (just installed it once, saw it requires Register Globals to be on and uninstalled it :-) ) I've seen a friend's website getting hacked 4 times in a row despite upgrading to the latest version. Could happen to any Open Source CMS though.
Joomla is more or less ok. Some things about it I like, some things I don't.
I made a website with it a few months ago, but had to correct a few mistakes at the core to get some modules to work as they should.
Joomla's got plenty of modules, but they're always a security risk, meaning that you might have the latest version of the CMS installed, but there could be a mistake at a third party module you're using.
As far as classic Amiga compatibility is concerned, I'm writing this on a Peecee :-( , but I like to see some sites working (especially Amiga sites) when browsing with my Amiga.
Writing an old standards front end for Joomla shouldn't be much of a problem, as you can override the modules' output. This way you can write the main template with the old standards and then override the modules you use to output accordingly.
There's a drawback though when overriding the output. Whenever there's an update, you'll have to compare the files that are updated with the files that you have changed and make the required corrections.
That's even worse of course if you've made changes to the core, or some modules' internals.
There's also going to be an issue with preserving content, but there should be components for PHPBB etc, although I don't know how correctly written or secure they are.
You could also check out PHP-Nuke (I think it was called), or see if you can override Xoops' output in its latest versions, as you can with Joomla.
To conclude, I can't really decide as far as the template is concerned. There should have been some CSS support for our old browsers at least.
Sticking to the latest standards will save you a lot of work, but on the other hand, this site is about people with outdated hardware and software (that's us :-D ).
It all depends on the amount of your free time and your patience.
P.S. You could also check the site's statistics for visitors' browsers to aid in your decision, or make two templates (one conforming to the old standards and one to the new) for people to choose from, or load automatically upon browser detection.
