Thickness of the steak is really personal preference. There is no "right" answer. The thicker it is, the easier it is to get a nice rare to medium rare in the middle with a good crust on the outside. The thinner it is, the easier it is to get the center well done without turning the outside to charcoal. I personally like my steak between 1.5 and 2 inches.
Spices are in the eye of the beholder. Personally, I'd just put salt and pepper on it. Depending on the quality of the meat, I may put some Worcestershire Sauce on it. Or maybe Soy, if it's an asian dish. Stay simple though. The flavor of the meat should shine through, so don't overly spice it.
Ideally you want a two level fire to cook steak on a charcoal grill. One side should be as hot as you can get it. Sear for 2-3 minutes per side over the hottest portion of the fire. The second level of the fire should be medium hot. Move the steak over once it's seared to finish cooking. Keep the grill lid down for the second portion. If you have a gas grill, just set one burner as hot as it will go and one on medium. Cooking over a medium heat will allow you to finish the steak without over doing the outside. Time depends entirely on how thick the steak is, temp of your grill, and how you want it done. Learn to tell "done" by feel rather than time or get a thermometer.
When the steak is done, you need to let it rest for 5-10 minutes. Otherwise, when you cut in to them, you lose a lot of flavorful juice. Resting allows the meat to retain the juice. Wrapping in tinfoil while it's resting will help it retain some of it's heat and be a bit warmer when you serve it.
Rust, or iron oxide, is not poisonous, unless consumed in large amounts. Thus it is relatively safe to grill on your barbeque.
What about the people who get cut by rusty nails and get lockjaw? That's not due to the rust, but rather, due to the bacteria on the rust, which is called, Clostridium tetani, which is found in the soil, and presumably, the nails have had come in contact with the soil, and so actually contains some of these deadly bacteria, and so, when a wound is made by the sharp object, the sharp object(nail) will also infect the wound causing tetanus, or lockjaw.
Which is totally irrelevant to your grill, as I assume, it hasn't actually come in contact with the soil has it?
So, it's safe, relatively
Best Answer
The idea is to clean the cooking surface. I typically fire up the grill, then, just before placing product on, use a wire brush to clean the grill surface. No reason that you couldn't do this at the end of a cook, preparing for the next cook. The advantage to cooking on a clean surface is that there is reduced chance of food sticking to the grill.