If a significant amount of towing will be in the mountains, seriously consider one of the turbo engines.
While the naturally aspirated engines will do the job, they lose power with altitude. At 8000ft, you will be down 25-30% on power. The turbo engines compensate by force feeding more air into the engine, so you lose almost nothing.
But really, even a naturally aspirated V6 will do the job for such a small trailer.
If you think you might upgrade to a larger trailer in the near future, it doesn't hurt to look at the 3/4 ton trucks. They aren't much difference in price and depending on what you find on the lot that meets your needs, it might even be cheaper.