Jon Goff gives a clear exposition on a first generation, single-unit (i.e. no in-space assembly required) cryogenic 30mT propellant depot that is doable in relative short order: Dual-Fluid Single-Launch Propellant Depots – Selenian Boondocks.
More info available in Propellant Depots Made Simple by Bernard Kutter (ULA) – Nov.10.10 (pdf). Several example BEO missions are illustrated that could be done with such a depot and no new heavy lift vehicle required.
Update: Jon says also
> I should mention that in Bernard’s presentation they also show how to do a 70mT version, using a 5m LH2 tank built into the PLF