I run a construction businuess.For putting posts up we usually dig a hole with a posthole digger about 4 ft deep, set your post in the hole and level it up, take a bag or two ofcement mix and dump in around the bottom of the hole, you can dump it in dry, the ground water will eventually turn it intocement.Fill the rest of the hole back in with dirt and tamp the hell out of it.It wont be portable but if your only on 2 acres you probably wont be moving it much anyway.

Run a 2x10 stringer around the posts, just attach the 2X10 to the posts withlags untill you have the outside of the posts completely boxed in.Then you can put youre floor in.Id use some 2x6 joist hangers, put them every 16 inches or 2 foot whichever you prefer.After you have the hangers attached you just drop your 2x6 into the hanger and nail it, much easier then trying to hold it up by yourself and attach it and will hold better.Your ready to plywood the floor now.Whichever plywood you decide to go with Id get some maxi bond or other type of glue and put down between the floor joists and plywood, since its outside the flooring will eventually loosen up and become squeaky if you dont.Then you can build your walls and roof in whatever design you want.

If it still seems shaky after the floors in just cut some 2x4 that will run at angle from the bottom of the 2x10 to the post on the other side part way down.Wish I knew how to do designs on the computer, It would be much easier to describe with pictures.