If it were mine, I would consider a hybrid powertrain based on a Sturmey-Archer hub. As others noted, skip the coaster brake -- remember the stores of "repack hill" in the early days of mountain bike development, when Breeze, Fisher, et al. quickly realized that coaster brakes were not up to the task at hand.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069