you are a lucky sod to get to build on that land... id go with a smooth flowy trail, get that gully dug out and you'll have yourself a sick snake run, with hip options and wall rides etc.

and i know they said it already, but dont build any wooden stuff until you have permission. keep it natural and small until its an established trail. no point wasting time building it all and it getting knocked down straight away!

If i were you i'd first get rid of all the grass, and then probably the top layer of mossy type dirt beneath. This will just make the trail quicker. I'd say the narrowest you want the actual line is 1 metre. Any less than this and you'll find yourself just following the dirt on the floor. You need a variety of possible lines which will enable you to get quicker as the track gets worn in.

Get the general line, top to bottom first, and then start thinking about where you want the jumps (if any). The steep section in the pictures may make a good landing for a ski jump, for example. Personally, when im building trails, i like to keep them as natural as possible, and let the roots and ruts come out. This way, you get a much more technical and difficult track.
I find it hard to "build" sections that are actually a challenge to ride because i'll build it how i'd like to ride it, rather than making it a genuinely difficult section. If you want to roughen the track up, try and dig through to the rock underneath - it's probably about 6inches under the grass. Or, leave a layer of dirt above the rocks and the most prominent rocks will start to appear after the track has been ridden a few times.

Don't use the fallen wood as materials for anything as beneath dirt wood rots very quickly, and your jumps will become a right mess. Although you might get away with using it for the backing of berms.
As for berms, don't just throw them in wherever you like. Ride a section time and again before building and a natural rut will appear. If you're skidding or sliding through the section, look for your tyre marks. Then go about building the berm along the line of the rut or skid marks.

Also, ride the track in the wet as much as possible when it's fresh, it'll bed everything in a lot more quickly, and loose dirt will get moved pretty rapidly. You'll then ahve a good idea very quickly of where and extra bit of support may need to go, or what have you.

What a ramble... i think i'm gonna go off to build. Haaha.

Cheers, Ben