Carl,
Sounds like a good plan in general.
It depends on the species, grade, and load, but I don't think your tie beams are big enough. Realistically, they're probably not going to catastrophically fail (plenty of barns have been built with smaller timbers), but you might as well do it right.
The rafters also seem to be too small and too far apart (intuitively). With metal roofing, either use closer common rafters with 2x4 purlins over, or principal rafters with closer common purlins. It's a lot lot more work fully sheathing a roof, and it doesn't really gain you much with metal roofing (maybe with condensation, but it's easier to just open the doors up).
Lastly, you can use either regular boards, or T&G, but I often use shiplap.
