I'm also in the planning stages of upgrading my fencing with similar issues. Hogs and coyotes are a big issue for me and I want to do everything that I can to keep them out. From talking to several owners of high fence properties, they set their corners and run barb wire along the ground to create a straight line for their posts. Then they install the fence a couple inches off of the ground so it lasts longer. Most fence will be Class 1 galvanized, which is OK. Class 3 has three times more protection from rust over Class 1, so it's more then worth the extra upfront cost. Keeping plants off of the fence will help keep the fence dry, which will also extend the life of the fence. I'm told that there are other products out there that do not contain salt like Round Up does, but I forget what they are right now.
I have goats and I found out real fast that they will stick their head into anything they can and get stuck from their horns. My fence will be 2x4 woven horse fence. I've wasted way too much time trying other types of fencing to learn that it's faster, cheaper and less stressful to spend the money upfront and get the best possible fence then trying to get by on something that costs less.
If sheep have horns, I bet they will stick their heads through a field fence too. When deciding between sheep or goats, I went with goats because they do so much better in my climate. I read that sheep look for a way to die. Sheep also cannot process zinc, so you have to feed them separately from horses. Goats eat everything. Goats are easy to sell, especially small ones as pets.
I have horses. That was my biggest mistake ever!!! But now that it's done, I have to deal with it, which is another reason for the 2x4 fencing. I currently have a hot wire around the inside of my fence, at the top of my fence, and at the bottom of the outside of my fence. The hot wire keeps the goats and horses off of the fence. I'm not as worried about them getting out, or breaking through the fence, it's the scratching on the fence that does all the damage. Once each horse touched the hot wire, they learn real fast and never do it again. My biggest girl actually jumped several feet into the air, and spun in the air almost all the way around before hitting the ground and running away. I've been zapped twice, so I know what it's like. I wanted to run away from it too!!! Goats are not affected by the hot wire like the horses are, but they still respect it. Coyotes, hogs and even racoons, all learn quick to avoid the hot wire. I have it about 6 inches off of the ground outside of the fence and we've actually heard a coyote touch it and start screaming as it ran away. Yelp Yelp Yelp for at least five minutes, until we couldn't hear it again. I walk the fence all the time and look at the tracks on the ground. It's real easy to see where an animal has touched the fence, they tear up the ground getting away from it!!!! Since installing the hot wire, I have not lost a single chicken to coyotes or racoons. I added a single strand around my deer hunting food plot a few yeas ago to keep the hogs out. Before, they would destroy everything, eat all the corn and scare off the deer. I tried shooting as many of them as possible, but never got enough to make a dent. The hot wire is six inches off of the ground and it's stopped them cold. the deer step over it, the hogs don't even test it.
My new fence will have 3 hot wires just like I have now. It's proven to work great. All my corner posts and gate posts will be set in concrete. My line posts will be packed soil, but I'll probably set them in concrete every 100 feet just for the added strength on long runs. I will drill all my holes with a 12 inch auger. I really like the one I bought from Tractor Supply that is made by Country Line. It's my third auger and the first one that actually digs through hard packed red clay.