The SLS is not needed, if only because the Falcon Heavy, perhaps even a super-heavy version, has a good chance to be ready before SLS.
Even better, stop relying on single launches of heavy launchers, and develop automated in-orbit refueling instead. A lot of the required mass is going to be fuel anyway; why not launch probes (or ships) with empty tanks on a parking orbit with medium launchers, and send fuel on several launches of small/medium launchers? Small launchers used often are bound to cost less in the long run than a heavy launcher that flies once a year.
On-orbit assembly could also work: either automate docking of the probe with separate fuel tanks or booster stages; or let astronauts assemble it at the ISS (with multiple launches, the penalty due to the ISS' orbital inclination can be overcome by sending extra fuel); or even astronauts at a dedicated short-term mini-space station made of a Dragon or CST-100 capsule docked to a SpaceHab or Bigelow module.
It's only a question of time before payloads (e.g. manned ships) outgrow even the heaviest launchers and require on-orbit work. Why not develop that right away?