The 50 x 100 beams may be a bit small for a 3200 span on 400 centers, especially if you anticipate any heavy snow. Upgrading to a 50 x 150 beams would be a little better but I have no idea what sizes are readily available to you. Adding more pitch to the roof (making it steeper) will make it less prone to snow build-up. A pitch of 30 high x 100 long would shed water very well.
Not sure how to attach to your wall, it would help to know if you're doing a post type of construction or if you will be digging a footer. The best would be a footer under the walls but even if you do a post style construction, the best bet would be to dig to the footer at the wall and set support posts on the edge of the footer. (if there is enough footer extended past the exterior of the wall.) Then you can attach the posts and the beam to the wall with one of several different style of wall anchors. The other wall posts should be set at a depth comparable if not deeper to prevent damage from the heaving of the ground during a deep ground freeze.
Overall, a footer would be a better choice of construction.
If you are going to put a floor in, you need to lay a vapor barrier. Heavy sheet plastic will work fine, then moisture won't be as much of a problem. Venting would be necessary considering you didn't mention any insulation.
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