Mercury is only 105 days away via a Hohmann orbit, but the delta-vee is nasty – 5.55 km/s from LEO then 7.5 km/s in low Mercury orbit. An elliptical arrival orbit only shaves 1 km/s off that, too.

Either way that’s a pretty tough ask for near-term propulsion systems carrying a manned vehicle, unless a multi-year space-soak of cosmic rays, ala MESSENGER, is acceptable… not!

Mercury has some advantages though. There’s radar evidence for large polar caps – much, much stronger signals than the Moon’s vague hints.