To build a typical 40″ x 48″ cart you would need the following shop equipment:
8′ box and pan brake. Most low end models will bend up to 20 ga stainless.
8′ shear or alternate would be a plasma cutter.
Turret hand punch or alternate would be a drill or drill press.
TIG welder. 120v is okay but for heavy welding you will want a 208v or higher. Some TIG welders come with a stick welding kit for arc welding.
MIG welder. Same voltage issue as TIG. Stay away from flux core.
Acetylene Turbo Torch.
Chop saw with steel cutting blades.
In-Floor hydraulic or pneumatic lift table.
18 ga 304 stainless steel is running around $225 per 4′ x 10′ sheet (New York). The thinner the sheet, the lower the price. If you are building a cart out of wood and just want to sheath the outside then go with 22 or 24 ga. Any other configuration I wouldn’t fabricate with anything thinner than 18 ga. If you have the money for hydraulic shop equipment and the slightly higher price for the steel, go 16 ga with a 14 ga top! If you are near the ocean you may want to request 316 stainless steel which is Pharmaceutical and Marine grade and highly corrosion resistant. 18 ga 316 stainless steel is running around $280.00 per 4″ x 10″ sheet.
Also mentioned was farming out your design to a reputable sheet metal shop. I have access to a full CNC shop which includes a Mazak Laser so I am spoiled. If things were busy though I would have to go outside, so personally I would look for a shop that has either a pneumatic turret punch or a water jet or laser. It’s far less expensive to have CNC equipment punch or burn the numerous holes required for rivets and other mounting than it would be to have a fabricator drill each hole. If you have or know of a friend that can put your design in CAD then you can save the fabricator a lot of time by providing them a CAD FLAT of all your parts. Basically, your FLAT programs are fed to their machine’s CAD/CAM software which then tells the CNC equipment what to do.