Wow, what a great thing to do. I hope the charity can afford to maintain it. The best thing might be to sell it and put the money in a trust. I would think it would cost $50K a year to maintain the property.
$50,000!
Funny!
$500,000 maintenance would be a start.
Cutting the grass will be $500 a week at least.
Replacing shrubs and trees that die out (not including seasonal changes) will be $15,000 a year.
Starting up and shutting down the sprinkler system will be $2000 a year.
Etc.