Four foot rolled field fence with heavier steel posts every 150' is fine for a working fence. It'll run $2.50 a foot installed. If appearance is the issue then the only limitation is money. The plastic, whatever it is, costs a ton and looks like plastic. Maintenance depends on your weather, but I can show you 100 year old wooden corrals that were only treated with linseed oil. If you use lots of “Penitrol” mixed with a fungicide and linseed oil for everything above ground you'll be OK for a long time if you seal the ends of the posts. Soak what goes underground with copper overnight in a garbage can, and when you set the posts mix the concrete instead of just filling the hole with dry redi-mix. Put gravel in the bottom of the holes and figure almost a small bag of redi-mix per post.
Just for looks, Home Depot has some ceder split rails that are cheap and look pretty good.

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