i think the lower you have it towards 58f range, the more traditional German lager characteristics you'll have, higher in the 65f range might be a bit more Bohemian, i.e, fruity in nature. remember that the yeast will produce heat of 5-10f degrees during fermentation, so you may want to low-ball the temperature to off-set added fermentation temps; perhaps start it at around 60f and drop it to 55f if your setup allows.
ja, to lager is to leave the beer (racked off from the yeast cake in the primary) in a secondary fermenter at increasingly low temps ala 33f as the ultimate temperature. W/ this particular style of yeast, i've heard both "yea" and "nay" concerning bothering w/ a lagering schedule. i suggest you experiment and see which results you like best w/ a side-by-side small batch experiment. Generally, all brews will "clean" up more if you lager them; the question is, is it necessary? only your personal preference will decide, so do a small batch experiment.
the darker wort/speise for an Oktoberfest isn't a problem. some chalk in the boil to stabilize pH and you're good!
just, if you're bottling this brew, be VERY careful about where you store it to bottle condition! it's a high-flocc yeast and therefore, by the time you bottle it (lagering or no) much of the yeast has dropped out of suspension. i've only used it once and i wasn't careful in this area and hence, got an extremely sour acetaldehyde ruined beer out of it. the taste tests of it (a Dunkel) pre-bottling were great but there simply wasn't enough yeast to properly convert the priming sugar into CO2 and hence, the plethora of acetaldehyde.