Well after posting my first response I read through the entire thread. sounds like you got real lucky with the property owner liking dirt bikes.
My advise now would be, don't go over board and piss the guy off, maybe start off with only one friend at a time, and seriously consider keeping it at that. No sense in ruining a good thing. Keep a good line of communication open with the land owner and always try to keep him happy. Its like having a wife, if you want to go riding, shes kept happy.
Any way, have fun and let me know if I can answer any questions about track building, water systems etc.....