your going to need more then this...
Even an abandoned parkinglot, is still owned by somebody even the city. your going to need to find out who owns the land you want to use, and get permission from them to use the land, a standard waiver excusing all liability of harm to the owners and all involved, will most likely be in order, Your'll also have to make arrangements to bolt some of the ramps down that can be moved, unless you feel like people stealing the equipment (yes it will happen.)
as for wooden stuff, your going to have to make sure its fully waterproofed and i'd advise covering it at night,
If the land belongs to the city, your going to have to petition them for a skatepark their, Ca is pretty chill about that from what I hear. and if they say no, you can always offer to buy the land, which will mean taxes and stuff on it, which means whoever buys it will have to pay for it, and will have to charge emission.
your better off imo, going out to the woods, with a shovel, some watering cans, and about 20 people and digging a dirt park, deep in the woods, where if somebody finds it, they most likely wont mess with it, and if they do, you can always rebuild it. dirt park/trails would also get use from more riders like mountain bikers, which will help you guys out on your upkeep so long as their not assholes.