The downslope for a start is good.
You might want to find out who owns the property before you get too involved with your time. Nothing's worse than working for months to get things going only to find them leveled one day.
Secluded park land and floodplain areas are the way to go.
You'll want to bury some big logs, big rocks, etc. to save dirt.
If you have the money and a dumptruck can get down there then I would go ahead and buy some good dirt. I don't know how good the dirt is there so that's always an option. You can build a pretty nice set of doubles with two loads of dirt. Time is $ and it takes forever to fill up 50 wheelbarrows.
A general rule is to have about twice the distance between the top edge of the landing to the top edge of the next lip as there is between the gap of the previous jump.
Therefore, a 12' double should have around 24' to the top of the next lip. (I've measured 9th street before and this is what works best.) You don't want to cram things too close together because then you can't make things bigger without moving what you've built and the pits between jumps feeling too quick.
Of course mix in some rollers, hips and berms in there somewhere.
Hopefully there is somewhere to get water too.
Also, try not to point the jumps toward the sunset. This is a problem on a few jumps at 9th street and you don't realize it until you've built things already.
Another thing you might want to do since it's level is to have a line coming back toward the start for obvious reasons.
I've been building stuff for a very long time so let me know if you have any questions