Giant Toughroad or Norco Indie might be good options for you. They're essentially classic rigid MTB's but updated to be 29ers with hydraulic discs and mid-tier components along with full provisions for racks, etc. The biggest risk is probably theft more than rust. The wheel size choice really comes down to sizing. If you're 5'8" or taller, probably a 29er would be a good bet, if you're below that, a 27.5" or 26" bike will probably fit better.

I've got a Giant Toughroad that I use as a commuter, but it can certainly handle anything but the most technical trails. Weight feels like its about ~23 lbs give or take. It's capable of doing just about any trail. Given your intended usage, a fully rigid bike may be preferable, since most budget forks aren't very good, and all forks require some maintenance.