Yea, I was thinking of going with fence posts or treated 4x4's. In may change my mind. I'm just cheap and don't want to spend the extra at the moment, although I realize it may be worth it in the long run.get yourself some creosote posts from the farm store for your main posts - keep the holes as small as possible and flair the bottom of the hole and concrete them in - and use treated 2bys on the ground horizontal and you can get away with untreated 2bys on the top horizontal. For your center horizontal you can either go with a treated sunk 2by or a metal Steel T post, sunk deep.
Those landscape timbers you are talking about using aren't meant for the vertical and curve as they go through the wet dry cycle and crack and split. Fence posts are center drawn and treated and are meant to take the cycles and the tension and weight of a fence.
As for the flanged hole and concrete, that's definitely the plan. I will be shouting for 6" diameter holes, especially if I manage to find an auger, and then I will just widen the bottom with a trench shovel.