Full fenders are the best thing you can do to protect your bike from rain. Get them long enough to keep most of the water and road grit away from your bottom bracket and chainwheel. As a bonus, you'll stay much drier yourself.
If you will frequently encounter road salt and sand... then all you can do is grease any threaded fastener you hope to someday be able to remove, and keep your chain reasonably clean and lubed. Salt and grit will trash your drivetrain sooner or later. Where I live, one winter is enough to destroy a chain.