I would certainly dig up the soil to make space for a proper depth of stone, also check if some drainage is required, this is the time to do it. I would also use at least 2 layers of the best weedblock material available. Some of it is good for nothing really. Also be aware there's a big, big, difference in the garden stone for sale. A bit like painting, people think you just slap it on the wall and there you go. A good landscaper is worth his/her weight in gold, same as a good painter.