I've made a few solid wood interior doors, never an exterior. The thing I would worry about in a solid wood door is keeping it straight. Even some of my "fake" solid wood doors in the current house (I didn't make them, just trimmed them) are not straight. Putting a clear skin over lower grade wood could help with stability but if they had used better wood for the inner layer, they probably would be straight.
So I would try and use quarter sawn wood if you can if you get a solid wood door. Certainly you do not want any irregular grain in the frame.