These things are for sale in alot ofplaces the prices are pretty much the same.
They are pretty sturdy ,in fatc I was amazed that after Isabel I didnt see a one that collapsed. I reccommend that you buy the snow braces ,adds a lot of strength for a little money.
If you are going to pour a slab for it be sure to allow a foot on either side of it so that runoff water hits the slab, If not be sure to buy the trailer anchors that screw in, They hold a lot better than driven down Re-bar. Also pick a good level spot for it.
There are two kinds the A frame and the one piece rafter. The one piece rafter is much stronger.