I'm no expert by any stretch of the imagination, but on my last one I wet brined the chicken for I think 6 hours (would have been overnight if I had time) and it turned out to be one of the best I've done. Seasoning was Dirty Bird by Cosmos which was very light - mostly for color since there was already salt in the brine. 375ish to IT of 165F in the breast. Thighs were at 175F if memory serves. Coals in baskets and drip pan in the middle. With the 18" you should be good with small amounts of coals on each side or possibly just one side. Maybe others will chime in here, I've only got a 22" roti. I've put onions and other herbs in the cavity and I really don't notice a difference. My vote is nothing in the cavity for a more even cook. We always place butter pats under the skin as well at the breasts. I've heard people say Kingsford Blue Bag is great for chicken cooks. I've used it with good results, although it burns fast and I have to add more. B&B orange bag lasts the entire cook easily for me. Basting optional of course.
Good luck, have fun and take pics!