We've moved into a house with a large garden that hasn't been
maintained for some years. The grass is in poor condition, and full of
weeds and couch grass. Part of our plan is to create some raised beds
and grow vegetables.

The plan is to strip the grass and import some top soil and manure to
build up the beds. Would it be OK to just invert the stripped grass ie
turn it upside down and put it back, and then dump the new topsoil on
top? Would the weeds just grow up through the new soil again, or would
this layer of old grass a few inches below the surface in any way
restrict the future growth of new plants?

Or would it be better just to dispose of the weedy old grass?

Thanks for any advice. Will.