Go with mahogany. It's strong, easy to shape, and holds varnish better than teak or cedar as it doesn't bleed nearly as much oil.
I replaced all the old wood on my 1960 DS1 with Philipine mahogany I found at Crosscut Hardwoods
http://www.emersonhardwood.com/cchw/categories.asp
Probably could have gotten it at other places but Crosscut had the dimensions I needed for the seats (1x12). It was all rough cut and under a hundred bucks, as I recall.
Look for "certified" lumber harvested from a plantation rather than torn out of a rainforest cleared for pastureland (my $.02).
Another possibility I'd considered was cutting/shaping the pieces I needed out of cabinet-grade plywood, then soaking it all in epoxy. This would have been expensive due to the amount of epoxy, but if you have a cheap source, it could be economical.