Also, regarding not getting a straight answer out of anywhere...Honestly, if you got 4 builders to make plans, you wouldn't get a consensus.
Unless you go down the path of putting down extension grade foundations (taking into account nearby trees, slope, clay type), filling a nice wide trench with MOT and concrete is going to be fine for a greenhouse.
What are the greenhouse plans, out of curiosity? I've always fancied building a wooden one.