For polywire, I use 1/2 inch fiberglass rod for corners and 3/8 inch fg or stepins for line posts. I avoid steel posts as they are a short looking for a place to happen. If running alongside an existing barbed wire fence you can also use standoffs instead of line posts. No wood is needed. Spacing should be whatever is necessary to maintain the desired height. In some cases that may be 50-60 ft and in rolling terrain, gullies etc it may be only a few feet. Would recommend keeping it simple.
Remember e-fence is a mental barrier not a physical barrier. It only needs to be hot and at a reasonable height to retain trained cattle. Don't over complicate it.