I did a heck of a lot of reading and research before beginning the project. What I found is that because the poles upon which the barn rest supports the entire weight of the barn, the need for a level foundation, as with traditional "stick wall" construction, does not exist. While I would not want to try to build on a steep hillside, a slight grade differential makes no difference so long as all poles are straight and level. The key thing is to find the highest point (corner) and to then determine all measurements from this point and establish level lines from here.
Depending upon how much change in grade there is, you may need to use longer pole for the lower areas in order to maintain sufficient wall height at all points.
I have decided to use Pre-made trusses for the roof instead of cutting rafters. This will allow for a completely open interior area of the barn and make the construction go a lot quicker, since I am working by myself.
Hope this helps. I am far for an "expert" but like I said, I did a lot of research beforehand.