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.

































A structural engineer would want to box my ears for this, but honestly...If it's a freebie like that I'd be tempted to just put down a couple of courses of concrete blocks, then mount it on that!
Though I guess a trench, MOT, and C10 is only a weekend's worth with a spade and a mixer.