One summer many years ago, I worked for a cement company mostly running the controls of a loading building that prefilled bins for loading the cement trucks. For your slab, be aware that there are a variety of cement mixes that have different strengths. The first thing to consider is the number of sacks of cement per yard of cement ( the more sack per yard the stronger the cement ). The cement is the more expensive part of the mix so as the number of sacks per yard goes up, the cost goes up. Typical sidewalk cement back then might be 7 and 1/2 sacks per yard, sometimes a little less or a little more. For bridges usually 9 or even 9 and 1/2 sacks per yard. And there are chemicals that bridge mix has added to it to make the final cement stronger. One of the chemicals slows down the cure time. Cement that takes longer to cure is stronger, than the same cement mix that cured faster. Another chemical makes the cement retain moisture longer while it cures. If a cement gets too dry while curing before it cures it will not be as strong as the same mix that maintained enough moisture while curing. Another chemical that was added to bridge mix made the cement have little air bubbles in it. That provides room for the cement to expand and or contract during freeze thaw cycles without cracking. Bridge mix had all three of those chemicals added to it. The company I worked for was near the borders of Pennsylvania and Ohio and sold cement to both states. Each state had there own preferred mix for bridges regarding how many sacks per yard it had, but both specified the addition of all three of those chemicals.
If you want to install a slab that is strong, and will last a long time, and have little chance of cracking, along with a good base, good depth, and rebar, a good bridge mix including those chemicals will produce a cement slab that will last a long time. The down side beside extra cost, is that you have to be sure to add enough texture to the cement because it can be slippery if you smooth finish it with a high amount of cement per yard, and if you end up with any high spots that you want to rent a cement sander to lower, it will be a tougher cement to modify once it is dry.
When I had to replace the cement walkway beside our house I ordered bridge mix with the extra chemicals.
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As for the structure, many areas have Amish who sell prefab structures like garages.
