I have a similar setup. The Nexus hub can be shifted while pedaling, coasting, or at a full stop.
Chain length can be a crapshoot. I got lucky and needed no tensioner or half-link. Plus, there is actually a little room to work with between the Shimano Nexus' axle and the dropouts. Between a half-link, the axle/dropout wiggle room, increasing or decreasing the tooth count on the front ring or rear cog, and the fact that you don't need an absolutely taunt chain, I see no need for a chain tensioner.
For stem length, it can also be a crap shoot. You can buy a cheap used stem on Ebay, say starting with 100 mm, and if it doesn't work, at least you have a gauge to determine the correct length.
That frame is a pretty smart buy. Having the options of rack and fender mounts is great. Longer chainstays helps eliminate your heel hitting any rear rack. And the wider seat stays and fork really opens up the options for wider, more comfortable tires.