Dig out the shrubs and topsoil. Place a compacted gravel lift with poly on top and put down the 2'x2' conc patio blocks for a foundation. Build your shed on top of that. You can use the conc patio blocks for the floor or just the perimeter foundation bearing areas.
For a 120 SqFt shed, you can make it totally portable and simple.
The last one I did had a plywood floor on 2x6" joists resting on treated 4x4" foundation. total cost was about $2500 and its built solid enough to skid onto a tow truck bed:
