Another option is using railway sleepers to make your raised beds. They come in lots of different lengths and should fit your project. For ideas try:http://www.railwaysleepers.com/projects/raised-beds-with-railway-sleepersand for some help in construction try:http://www.railwaysleepers.com/blog/tips-advice/how-to-build-a-raised-bed-with-railway-sleepersAll the best.
I'm sure you've designed your raised beds already, but if not, have a glance at some of the shapes possible!http://www.railwaysleepers.com/projects/raised-beds-with-railway-sleepers
I can't add much on the soil debate, but here there are a few hints on height & construction etc... for making your raised bed which might be useful:http://www.railwaysleepers.com/blog/tips-advice/how-to-build-a-raised-bed-with-railway-sleepers
Hope it all goes well.
I've been looking at building some raised beds and felt short of ideas and came across a website with lots of raised bed projects and lots of other stuff! Might help!http://www.railwaysleepers.com/projects/raised-beds-with-railway-sleepers
Good luck with your garden!