Personally, if I think it's going to be even damp out, I use White Hair Glue. It stands up to just about anything, though does take a more vigorous scrubbing of shampoo to remove later. If it's going to be dry and cool, I use Ben Nye or Mehron hair white with a little silver mixed in for a slightly muted tone to it. Using Final Seal on it (Yes, on hair) does add to the hair white's resilience to the elements a fair amount.
Wig-wise, I've used one before and it cost about $80. I didn't have any problems with it, I've had it braided and unbraided before. You just need to be careful in its upkeep and it'll last plenty long.
And I am a strong supporter of hats, as well. Preferably large-brimmed fuzzy ones.