shed concrete

how hard would it be to mix up 5 cuboc meters of concrete DIY style? I have a mixer myself with the option of borrowing up to two more. What kind of quantities would i be looking at (sand,aggregate,cement?) Also i have had a suggestion to reduce the thickness to 75mm, that way reducing mixing time. Im a bit hesitant due to the site classification (clay) If i were to pour the shed in a few parts whats the correct way of finishing the edges for later joining, without obvious patching? ...

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