Hi Newbie
I would try and avouid putting down a concrete base, unless you are good at levelling and then screeding it towards a drain that you will have to put in the floor too.
Dig out the stump, then you have a couple of options. You can lay a path up the middle and have raised beds on either side (you can buy enough topsoil for that size greenhouse fairly cheaply), or remove what topsoil you can. lay a permeable membrane over the floor area and cover it with gravel or chips or similar, then grow in pots or growbags.
You maight also want to consider staging down one side to put your seed trays, small pts etc on - and for storing all the other bits and pieces that you will surely gather 1

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Rat

British by birth
Scottish by the Grace of God

Blog updated Wednesday 17th September