I've set up a number of bikes with an Alfine and a double crank, using the Shimano chain tensioner. It's important to use a two pulley tensioner like the Alfine, Rohloff, or Paul, or even an old rear derailleur, and not a single spring loaded tensioner like the Surly Singleator.

I have an almost-new Alfine hub and have ridden it without issue on a couple of stupid cold days. I haven't heard any reports about hubs with more miles on them in colder weather though.

With drop bars you need something like the "Hubbub" adapter which installs into a road bar-end, and gives you an extension the proper diameter so you can mount the twist shifter. Some people like to cut down one side of the handlebar so it looks a little better. Works great, and we've sold a countless number of bikes set up with one. Unfortunately JTek isn't selling their bar end shifter right now.