in general, aquaponics wants to avoid metal in contact with fish water. Aquaponic water tends to corrode most metals quickly and many metals contain things that would be bad for fish. Food grade stainless steel might be an exception but I've never heard of a stainless steel stock tank.
You can coat the tank (aquatic eco systems sells coatings that would work) or line the tank but if you are trying to save money I would simply get the rubbermaid tanks unless the metal tanks has been given to you for free.
As to the liner. I've used plenty of EPDM and cycled up fine. The important thing to know is not all EPDM liner is created equal. Some less than honest people will get cheap EPDM that is meant for use as roofing liner and sell it on e-bay as pond liner, not good, the roofing stuff is treated with chemicals to inhibit bacteria, algae and mold and it will inhibit cycling.
The firestone pond guard EPDM pond liner works fine and is the most durable liner for rough situations. If you are filling a bed with gravel, I'd use the EPDM if you must use liner.
Now if you are going for organic certification, then it makes sense to use materials that others have managed to get certified as well but for a home system I'm not sure if you care about that.
Stainless steel may or may not be all that good. Generally stainless steel isn't supposed to rush or corrode but not all stainless steel is created equal. Do you want to risk your growbed rusting? Sinks are generally not left full of possibly corrosive materials long term. The bottom of a grow bed is usually in contact with the most corrosive liquid in the AP system most of the time.
Might be better off saving the stainless steel sink for your outdoor kitchens and fish cleaning activities.
You can line a galvanized or metal tank with a food grade safe polyethylene (LDPE and HDPE)liner. The same for the grow beds. Many substrates for tanks have some drawback or other if not created specifically for food and fish grade. For instance, concrete can make great tanks or grow troughs, but the lime in concrete can really mess with ph. Again, a simple and safe solution here is liner! Check out this link for a list of Polyethylene tank liner suppliers http://www.thomasnet.com/products/tank-liners-44362309-1.html Also, Aquatic Ecosystems sells tank liners, but they are made of fish-grade synthetic rubber (ethylene propylene diene monomer). http://www.aquaticeco.com/subcategories/962/Sweetwater-Pond-Liner/ However, I am not sure if this substance would be organically certifiable and therefore perhaps questionable for food grade. Don't quote me on that one though! Also, EPDM has been known to make start up in an aquaponics system difficult. Hope that helps!
aqualined -with a pond liner?
A fish safe, food or potable water safe coating that will stick to the metal and protect it from the fish water would make it suitable for use. (Most of the pictures of galvanized tanks in use for aquaponics are aqualined which I believe is a fish safe/food safe coating.)
I believe the aqualined tanks were a professionally applied coating. This was something common on some of the older flood and drain aquaponics systems in Australia.