Raised Beds

That sounds a great idea, Dan. Before you start the actual construction I would suggest that you cover the base of the raised bed with porous membrane, not so much to stop weeds coming up but to stop your soild going down into the gravel.
The choice of materials for the walls is then almost unlimited. The easiest to work with would probably be timber but then you would be restricted to square or rectangular beds. Bricks can make nice raised beds but my prefered material would probably be stone, local stone for choice. Is there an architectural scrap yard near you. They often have a large range of stone and other materials that may interest you.
Our 'local' stone is rounded boulders from farmers fields. When I am building a raised bed I lay one layer of stone right round the perimeter of the bed and then fill this with my compost. This gives me something to lay the second layer of stone on without fear of the stones rolling away. I continue like this to the top of the bed.
Good luck with the project and give us some picture blogs as you go along.

I can happily say that B's method works - we have several raised beds plus a double dyke at the front of the house all built by him and all standing strong.