Are you planning on only fencing the pasture, or the treed area as well? Depending upon where you live, there can be quite a lot of good forage in a stand of trees. That said, debris and branches in the trees can increase the risk of injury. If the trees aren't too thick and you can clean up the ground around them and trim the lower branches, you may be able to use that area too.
As for the one acre, you won't have a lot of grass with 2 horses as they will first graze it down and then chew it up with their hooves. You could fence off a sacrifice pen where your horses spend most of their time to help the grass grow but you will need to provide regular exercise for your horses. You will still need to seed, fertilize and harrow regularly. Even doing this, you will really only have "treat grass", not enough to sustain a healthy diet.
Fencing material depends again upon your area and your needs. Smaller spaces like sacrifice pens need sturdier fences. If you are fencing in the trees, you have to check regularly to be sure a branch or tree doesn't fall on and break the fence. My preference is to use good treated wood posts with a safe fencing material such as wood poles, wood planks or electric rope.
As for manure, we use a quad with harrows to drag, dry and spread the manure in summer and a good old fashioned manure fork and wheelbarrow in the spring and fall. In winter we let the snow cover it and do a big clean up in spring where we pile the excess.
I hope this helps a bit.