According to Sheldon Brown's site, the bearings and races etc. of the Nexus Red Band hub are supposedly Ultegra-level quality. The Alfine's are supposed to be even better! Where would that equate it? Dura-Ace?
You have to get anti-rotation washers specific to your frame's dropouts in order to prevent the axle wrecking your dropouts due to the natural torque stresses on the frame. If you have used an internal gear hub before, like those good old Sturmey-Archers, this should make sense.
As for chain tension, horizontal dropouts or track fork ends would be ideal, but if you have vertical dropouts, you can use a chain tensioner as stated above, or you could do as the fixed gear freaks do: calculate a "magic chainring to cog ratio" where the chain is tensioned properly in the absence of horizontal dropouts and a chain tensioner.
Straight chainlines are pretty