- a propulsion system that can send an habitat and supplies good for, at least, 6 months.
- a separate trip to send the supplies for the return mission and park it in Martian orbit for rendezvous when the manned vehicle arrives
If you also want the crew to land on Mars, you'll need a lander. You may consider doing the surface-to-orbit return mission on a separate vehicle you can land before. You may consider sending supplies and a shielded habitat (Martian surface is about as shielded from cosmic radiation as space itself - not at all) if you want the humans to stay.
20 years is a very hard schedule for that.
You can send the unmanned parts with current technologies, but the manned part depends heavily on the development of adequate shielding and/or adequate NTRs to make effective use of the reaction mass the spacecraft carries.
If shielding is developed before, you could even make the whole thing easier - you put a shielded habitat in an Aldrin cycler orbit and just send crew and supplies (and spares for the habitat) to rendezvous with it when it passes by Earth leave the habitat when it's time to enter Martian orbit. The trip is longer, but it's easier if you don't have to give the whole habitat the required delta-V.