Rip out the old slab, the subgrade should be pretty good, so don't dig it up unless you have to.
Build your forms, make sure elevations are good, 4 or 6 inches from top of form. If you do a 4 inch slab, you my want to thicken the edges. (turn down)
Get yourself some #5 or 6 rebar, tie it on 3 ft centers.
Go down to the newest housing development, look for the guys who are actually pouring the slabs, the guys who actually have concrete on them, not the guy standing around or sitting in PU, barking orders.
Pick the smartest looking one, take him to your project, ask him how much. (Before this step call a concrete contractor and get an estimate, so you will have an idea of what it would cost).
Concrete is ordered in yards (27 cubic ft to a yard). 1 yard will do approx 81sq ft 4 inches thick, so go figure, literally. Or just give the dimensions to the batch plant, they will tell you what you need, including PSI.
All concrete cracks, joints limit the amount of cracking. Your subgrade has been under a slab, I assume for awhile, so don't sweat the subgrade.
When they pour the slab, you want to make sure the rebar gets pulled up into the middle of the slab
If this is a shop (auto etc.) have a broom finish put on it, you don't want a glass smooth slab, you will be falling on your a** everytime it gets wet or oily.
The guys doing the slab will do whatever the weather dictates, so don't sweat that either.
And if I sound like I am full of BS, go down to Home Depot/Lowes and buy a book.
waterdog