Nothing with a Mahogany door in Mn as long as it is properly sealed and r=the finish is maintained correctly (just like any other wood door on any other climate).

I went and saw the door, and it is really gorgeous, solid mahogany. I am just worried it will get ruined by plenty of moisture and sun. Would I caulk all the exterior joints and seams with a silicone or something else? Read a little about using tung oil and spar varnish, that might work I guess. It is about half the price of anything else I can find and is a perfect fit in the current opening.

As far as a wood door, mahogany is by far the preferred material for good service. But in severe exposure, it will still require that the finish be renewed periodically. Clear finishes in the sun do not last very long.
Casey