Congratulations on your purchase!
I have and 8'x10' greenhouse. I have shelving down one side and the other side is a big bed in the ground. I have pavers down the middle and under teh shelving. (Actually not quite, pavers are slightly to one side to leave a bigger bed on one side - it is about 4' wide, path 2' wide and work shelf 2' wide.)
The big advantage of having a ground bed is that your plants have a much bigger root run. Also some water from outside can permeate the soil beneath the bed so reducing extremes in moisture content that can trouble grow bags. Also all the minerals in the ground are available to your plants.
The disadvantages (always quoted) are the build up of soil pests and diseases. I have had no bother with this. I have enhanced the soil considerably with mature manure, and so on but have never removed the soil and replaced it as some books recommend! Of course this is only my 3rd season....
However my mum has been growing toms in her greenhouse for 30 or so years with no problems. I think she did get someone to dig it out for her and do a complete replacement exercise once or twice in this time. Believe me she grows superb toms!
One other consideration is keeping couch grass roots out of your beds in the greenhouse - they are very good at growing under even a concrete base. The width of a flag stone however seems to stop them - we have just this year put a paved path down the outside of the greenhouse - so I'm hoping to have less of an invasion this year!
Good luck with the assembling exercise!
Sally