Considering that Super 8 doesn't record audio, doing a Documentary in that format would be quite difficult. It would certainly make for an nice look, but the sync problems would be enourmous. Hi8 is the predessecor to Digital8 and MiniDV - if you can use MiniDV or even Digital8, do so. But don't let your camera limitations stand in the way of making a great video.
Something to consider about Super 8, though - I'm interning this summer at a local film school/camp in their Super 8 unit. I did it last year, and the way they do it actually makes for quite an interesting and unique experiance. We shoot with crappy old Super 8 cameras, hand process it, and digitize by projecting it and recording the projection with a MiniDV camera. The end result is a terribly choppy, grainy film - but the look is unlike anything I've ever seen. It's experimental filmmaking at its best. The only downfall of the process is that it's expensive - Super 8 film isn't cheap, and if you want it processed well it can be pretty costly too. Locally, we have a three-week turnaround for Super 8 processing, since it has to be sent to Switzerland. Hence why the school does hand processing. You can also use blank film and do some pretty cool scratch animation and effects with it - though you'd want 16mm+ for that, and that requires another projector.
All that said, Super 8 is great fun no matter which path you take with it.
