Are you pouring foundations or just standing the wood on the ground? One of the things I thought of when I fantasized about building one of these was using 6x6s on foundations for any heavily load bearing parts (valleys and turns) and running steel I beams across the ground with 4x4s for lesser load bearing parts (like hill tops, the straight part of a lift where iirc there is less torque on the structure than at the bottom or top), similar how to they bridge crossovers in real life. As long as your I beams are well braced so they don't shift (maybe a foundationed 6x6 every 4 bents or so) this should cut down a lot of cost and labor (pouring cement absolutely sucks). Calling local scrap yards to see if they'd be willing to sell I beams at slightly above weight cost should save a lot too.
I would also use 1" for tracking rather than 2" like on real coasters as it will be significantly easier to work with at smaller radiuses you're dealing with. A good set of clamps and an air nailer/compressor to run it are what you need here.
If you're serious about this, don't get hung up on things like layout and how big your drops will be. That's trivial at this point. Worry more about the practicality of actually putting it together and having it not fall apart. When I was younger I used to BMX bike and after we'd watch videos we'd go try to build ramps and have a skatepark in the driveway. It's a lot harder than it looks.
Edit: err, not I beams, standard channel is what you want. Heavy gauge. http://www.turnersteelcoinc.com/html/s_standard.html