as I understand it, the range of gears top to bottom is similar to the usual triple chainring and 8 or 9 spd cassette, am I right? I appreciate the steps between gears will be bigger, but there are a few 'redundant' gears in a derailleur drivechain setup, n'est pas?
not with an alfine – i think its more similar to a wide ranging single ring set up.
to get a full range of gears (nearly) you need a rohloff, which currently new will set you back around a grand.
however, they work flipping brilliantly (ime)
tensioner wise, there are sprung ones – best i've found is, again, a rohloff one, but its near 50 quid. there needs to be throw in the jockey wheels to allow for chain growth on a FS – therefore regular ones such as DMR tension seeker and on one doofer don't work as they don't allow for enough change in chain tension.
this can be overcome in FS' with a pivot around the BB, such as a kona A (i think there is a knolly too, and a couple of others) whereby the pivot around the BB keeps the distance between the cranks and hub constant.
HTH