I am more handy w/ wood than metal.. All wood will rot over time... with the exception of cedar or teak... which is why its so expensive. If you were to use such wood - you would probably P*ss off a lot of woodworkers for covering up such beatiful wood. So you can use pine for your structure, which will rot, and will still be expensive & heavy. Not to mention you will have to plane it to get it perfectly square. I think you are making more work for yourself.
Long story short- forget about the wood. I think automark is on the right track w/ the pre-manf. treads. Or if you want to go completly hand built, I would think if you were to make alumn. 2x6 "boards" the strenght added by bending the edges over 90 degrees will add ALOT of strenght. put some crossbraces underneath and it should be ridged enough to hold up. I wouldn't "use" your bed. but it would be fine for show.
