Hi
composite posts are the way to go. Metal tred in can help on corners.
The poly type wire is better where you have dear etc. It does not break like some of the galvanised will. Not so good when you get wet snow, then metal is better.
If you do not want problems at first turn out then you can train them to your new fence by placing a training fence a few feet inside their current location.
Best places to ground to are water, low wet ground, steel building frames etc if they are handy.
Buy the best energiser you can afford. The more power the better, and those with multiple power settings are good. If your cows know the fence the you can use low power, or hig to train them, or high to burn through growth of grass, weeds etc.
I would use one strand for cows, two if their was young calves their as well.
1wlimo