You don't say how large your greenhouse is or what size slabs(Pavers) you
are going to use.
Why not sand to level then weed barrier then slabs.
I would be more inclined to lay weed barrier, then just lay a 1 1/2 to 2inch
layer of cement to make the path, and the same depth of gravel under the
benches.
What ever you do you will always have a few weeds but the pull easily
enough. and some plants will love the atmosphere under the bench.

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk