Since you'll be wearing slacks, shirt & tie, or a suit, or a suit with a rain coat, you'll need something that will allow you to be comfortable. A more upright position is generally desired when wearing these kind of duds. A drop bar, road bike or 'cross bike will not be comfortable for you.

My son, who is now serving in Boise, Idaho has a mountain bike. He is on unpaved roads sometimes, plus he already had the bike prior to his mission, so this was a no brainer.

If you know for a fact that you'll be on paved roads 99% of the time, I'd go with some sort of hybrid. Get a rack, FOR SURE. Fenders would be highly recommended.

I served in Taiwan. My bike was a completely hideous, old gas pipe, rod brake, 50 pound behemoth. Bullet proof, for sure... but it got me everywhere I needed to go!

You can always call the Mission Home where you're going to serve and ask about the road conditions that you're likely to encounter.