I plan to build a 10'x12' shed in my yard. I have this patio area made up
of 7-8 rectangular slabs of cement, all with different surface and each
slightly tilted so that I have this uneven patchy surface.

What I want to do is erect a shed over a large portion of this patio and I
am trying to decide if a cement or wood floor is best for the floor. OK,
wood is easy, I just need to make some footings. My question centers around
the cement pour.

Do I need to remove all that ugly patio cement and start with a new slab
poured onto compacted earth or can I (and this is the real question) pour a
new slab on top of the old cement. I suppose I would need a comcrete
bonding agent and either rebar or a metal fabric to reinforce but is
building up like this a recipe for cracks or other problems. The old patio
is probably 30years old and is unlikly to settle much more. What minimum
thickness should I use?