Turning a huge parking lot into a garden

I might be working with a non profit next year. The NP is in an old movie theatre with a huge parking lot that they own outright. They would like to turn it into a community garden space. I'm wondering about the best way to deal with the dirt. Right now the plan is to cut up the asphalt 20,000 feet a year for the next five years. Ideally I'd like to just cover crop the space for the whole year to work on rejuvenating the soil and aerating it a little bit but I think they might be more eager than that to have a functioning garden next year.

I guess I'm wondering if anyone has turned over parking lots and outside of major compaction what the big dangers are.
Also can I bet on the soil being totally toxic from the asphalt? I'll get a test? I have access to free wood are raised beds the way to go?
Also I'm going to be cutting up the asphalt in a grid with a walk behind diamond saw. Are their good ways to reuse big square slabs of asphalt?
I was thinking of building compost insulation out of them but figured it would be a bad idea with the contaminates in the asphalt. Any thoughts?

What about just cover cropping in between the raised beds?

They want to get a 50,000 gallon cistern and feed it with rainwater. How much is something like that?

Thanks Permies,

Hendall

For a tank that size you might need to get a custom quote. Not sure where you're located, but here is a source of very large tanks: http://www.water-storage-tank.com/corrugated-steel-tanks.html

Personally I would avoid using the asphalt for any purpose where it might contaminate food-growing soil or compost. I would use it only in areas for decorative plants which won't be cut as mulch for the edible plants. A very active fungi rich ecosystem might render it non-toxic, but personally I would be concerned.