The joints are all just bridle joints. Most woods in my opinion would be strong enough. I'm always partial to hardwoods and honestly, the base will make or break the whole piece. IMHO, if you're putting it on Douglas fir legs, it's broken. Not for the structural integrity, but the athletic.
Either dark tones and straight grain or a contrasting wood like red jatoba or even carefully color matched Maple. The base you linked for inspiration is beautiful and should be relatively easy to execute given proper care is given to setup of the cuts in a few critical cuts.