If you wish to promote bio diversity, get in touch with a good landscaper (or recruit some local residents) to build a pond in the land. Put down some meadow mix (a grass seed mixture that comes in different proportions but gives a natural look. For the areas where you want plants hold a conferrence regarding what birds and animals you would like to encourage to the area, then look into what insects they eat or ruffage they like and what plants attract the insects. This is not an instant garden and will take some years to fully mature into a diverse haven for whatever creatures you would like to encourage. For the maintenance as it is held in trust I would try and create a commitee out of local residents - if no one is interested or has the time then look at other community groups but you will find that once it has taken hold, nature has a way of taking care of herself!