I forgot to mention the boxes will be watered with a drip system.

Raw steel will start rusting within days if wet. Sure, it'll last quite a few years, but it will rust heavily, and you don't want that in your beds. The best thing would be a baked on powder coat, but obviously that's not something you can do. So, if I were you, I would head to a hardware or big box store, and find some cold galvanizing zinc spray. Grab a few cans of that, and give it 2 coats. Also grab a few cans of outdoor rust preventative spray paint, in the color of your choice, to give it a nice look. I wouldn't worry about the paint, because if done properly, it won't flake off or leach into your soil. Insulating is your call, but not necessary. It'll stop temperature fluctuations, and keep the soil warmer in fall, but it will also keep it cool longer in spring. Also, if you're not too familiar with the proper way to spray paint something, feel free to say so, because then myself and others can give you some pointers.

Joe

how about horse troughs instead ?

Thanks Joel. Dickie, I do like the look of horse troughs, but I need something that will retain about a 24 ft length of my yard. That's why I decided to go with the custom boxes. I am now just looking for a lining or coating that will protect the boxes, but that will not be toxic to soil/vegetable/me :)

back in my bridge building days we used an epoxy coated rebar. i dont know if epoxy is toxtc after its cured but it last forever.