Things were beginning to converge and congeal up until last week, but now they have begun to diverge again.
First, I was settling on a solution using an Ubuntu/Xubuntu command-line system as the base. But I am running into some difficulties with some of the non-standard ways that Ubuntu/Xubuntu handles some things--having no real "root" for starters. This is making it harder to get e.g. sound going in this context.
So... now I am working with a minimal Debian install in the hopes that it will be less irregular and more completely documented. (Ubuntu/Xubuntu doesn't need to document a lot its inner workings because they wrap those workings in GUI goodness. It's only to people with geek interests like me that it matters.)
Another alternative I am considering is starting with a full Xubuntu install and then stripping away the heavy bits and replacing them with lighter ones. But I think I am going to try working with building up a command-line based Debian install first. If I can get this to go without too much pain, I think it will make future maintenance a lot easier.
Second, I was settling on IceWM as a window manager/desktop environment solution because it does both in one swoop, integrates some widgets as well (CPU load, etc.), and does it at a very low cost. But I am having a hard time accepting the way it structures its program launching menu. The default setup is kind of klunky and modifying it involves hacking a text file. Also, even though the whole package is theme-able, I have yet to see a theme for the taskbar that makes me really love the visuals. It's a good solution for sure, but I am wondering if there is better.
One option I have considered on-and-off is Fluxbox. It's configuration isn't that awful mostly because there are nice GUI tools available to help you. But the WM's behavior doesn't feel 'right' to me. For example, the right-click nested menu options are tweaky to navigate no matter what theme you use. I have yet to figure out exactly why this is so, but I suspect the nested menus are a bit too eager to let go of their states. Also, while I think Fluxbox's panel looks really neat, I don't like the way open windows are dynamically sized on it starting from 100% for one window and then shrinking linearly. Finally, to make it work with ROX-filer (the leading contender for file management) you have to use ROX's "Blackbox hack." And even then I've seen one or two weird things happen. Still, it's worth keeping an eye on.
The option I am currently most seriously considering is a combination of Openbox and fbpanel. Openbox is fast and lean; fbpanel can be made to look good fairly easily, isn't a total pain to configure, and handles default menus in its program launcher better than IceWM. fbpanel isn't as lean or as widget-full as IceWM's taskbar, and I don't completely love Openbox's context menus or the poor frame grab hints, but the combination is a credible alternative to IceWM and is worthy of further exploration.
I looked at PerlPanel and PyPanel as alternatives to fbpanel. PerlPanel is awesome in the configurability department, but it's quite a memory monster and it no longer seems to be actively maintained. PyPanel was just weird. Maybe I need to spend more time with it, but its default setup was nearly unusable with Openbox+ROX.
I also made an Openbox+ROX-desktop detour (using ROX-Session) but had setup issues that no end-user would want to have to deal with. I love the ideas behind a lot of what the ROX project is trying to do, but I am not sure they've got the implementation quite right yet. So unless ROX's desktop setup gets better organized, I am not going to further investigate that route right now.