I looking building some outdoor furniture and looking for suggestions on suitable timber species and profiles.  I'm aware that I could simply buy furniture for less money and trouble, but I specifically want to build some.

It's early days but the basic concept is to build diner-booth style seating with high backs and boxy bases. The bases will be used to store cushions etc. The seats will sit on a concrete pad but will be exposed to rain, wind, sun and frost.  We are in a coastal salt-spray zone too.

I'm planning on cladding the bases with closely spaced planks (clears) over an internal timber frame.  The planks will do some bracing but the main structure will come from the internal framing.

Some water will get in the gaps but not much and we are talking outdoor cushions which are water-proof.  I want to protect them from UV and blowing away.

So what timber?  I'll use treated pine for the inside but I'm undecided on the planking.

I was thinking some sort of hardwood (FSC certified).   Having look at M10 this seems to be available as <25mm deck planking.  Choices included garapa, kwila, teak, mahogany and purple-heart. 25mm seems light to me but I have almost no experience building with hard-woods. 

Stepping away from hard-woods, western-red-cedar would be another possibility.

I'm unsure what would be a suitable thickness in any of those timbers to avoid warping cracking etc.

I would appreciate some advice.