Pressure Treated would work, I guess, if you can handle the rounded edges, the green colour, the smell of the preservative, the toxicity of the sawdust you'll be breathing... and that's if you can find straight product. The impregnation process can distort the product quite a bit, I've found. Certainly I can see using it for the base if you are going to lay it directly on soil or concrete, but there must be a sawyer or two in your part of the world that could cut the few pieces you are going to need.
Take a look here...
http://www.forestryforum.com/board/index.php?board=11.0 Check out the Timber Framing thread on their site 'moderated' by Jim Rogers... there is an excellent thread on there about the tools and techniques you'll by needing and using for your shed regardless of the size and material chosen.
Best wishes for the project...
