I would think (without doing calcuations) that a 20lb tank might be a bit on the small side. Without know what you intend to use for fuel, it's hard to tell. But 1/3 the size of the cooking chamber seems to be the norm. If you are planning on using splits, you will probably want to go bigger on the firebox, charcoal.. you might be OK.
Expanded works great for grates.. get the flattened type to make it easier to move meat around without tearing it up and get the thickest you can. I saw 9 gauge the other day online, but I don't know what you have available. I use thinner stuff myself, but it's welded to an angle iron frame for stiffness.
To keep it from rusting, as BBQ Eng said...use it. Once you get your first cook or two done in it, it will be seasoned enough to keep it from rusting. If you are going to clean it, don't be anal about it and clean it down to metal....and only clean it just before you cook, not after.. That's an old trick taught to me by a guy who cooks more meat on a weekend than I do all year. He lets the whole cooker, grates and all sit all nasty until he's ready to use them the next time them he hits them with a steam preasure washer just before using them. Metal can't rust if it's coated in greasy crud.
Oh.. and another ditto from above... plain carbon steel grates.. or even stainless if you have a way to weld it, but stay away from any kind of coating...