Thanks for all the input, and what seems to be general consensus on the durable woods that could be used. I had considered pressure-treated wood (I assume that is what Stephen Thomas is referring to as "PT"), but I was concerned about the possibility of having edible plants/vegetables growing in that environment, and whether there would be consequences of some sort. I would also assume that the self-treatment could carry some risk, but feel free to disabuse me of that notion. I used some of that stuff before on a few fence posts I put in the ground with concrete footings, and I seem to recall some warnings on the can about some level of toxicity.
I had not priced teak at all, and figured it would be expensive, just not "Holy Crap!" expensive. Ah, well, maybe I've saved the life of a saw blade for now. Building something like this from "permanent" material would fit into part of the current less-consumption philosophy, as in "buy quality, and less often", but of course it does nothing for the supply end of the equation where the teak gets harvested...
I will check out the synthetic deck materials. That looks like it might be a fine application, although I seem to recall that when I priced that stuff some years ago it was good deal more than I had hoped, for whatever the heck I was thinking about at the time.
Regarding the joinery, on the previous editions, I used 2x4 corner reinforcements on the outsides of the corners, with Titebond exterior glue and standard drywall screws, with a center rib support underneath and at each end, and still got a lot of bottom plank distortion over time, so I'll chalk that up to water vs. wood, the age-old battle. I'll need to consider better engineering for that aspect. I would love to make finger joints at the corners, but I'm 68 already, and I don't think I'd get through the project before I croak, and certainly not by this Xmas.;-)