I'm trying to keep dairy heifers and dry cows inside the fence, so my experience might be helpful, might not.

Wood posts on corners and tops / bottoms of hills.  Round treated wood posts, 6-8" is what we usually use. 

1.75" timeless posts every 50', wires through the top hole and 15" down. Most posts are 5.5' but have some 6' to help in low spots to even out terrain.

Wire is 12.5 ga high tensile, just pulled snug by hand, still has a little bit of sag. As long as it is off the ground I'm happy. No braces on corners, they haven't moved because there is little pull on them from the fence wire.  First fence built like this has been four years of freezing and thawing, wet and dry and the corners are still pretty decently plumb.

Last but certainly not least - a HOT fencer so the cattle know not to mess with the fence. Most touch it once, slow learners might need a second lick of the wire but they respect it after the initial encounter.  It probably helps that Holsteins are big wimps, for the most part.