You have to have solid corners to keep your fence up. Use extra large corner posts that are deeper than the others. Whatever you use, such as wood, space out the length of a post and put down another one in each direction. Then tie in a post about 3/4ths of the way up as a brace. Then use wire cross braces to keep your corner tied together. Space out your interior posts and use a fence stretcher to pull it tight.
The extra post in each direction with a post spacer and cross tied with wire really firms up the corner. You should be ok with 300' but for longer runs, you would put down 3 large posts again in a straight line with post spacers and cross wired every so often. This helps keep a long span tight. Drive around and look at farm fences or look at fences along the interstates which were built to last and keep animals out. Good luck.