Congratulations on grabbing one of the best backyard BBQ pits made Tom.
Lower cooking grate and charcoal grate are not the same as the ones from an 18.5" kettle.
I learned to cook on the WSM using water, but if you've cooked on a UDS you may not need to. Foil the inside of the water pan whether using water or not. If no water, a few golf ball sized foil balls in the pan before you foil will help keep the rendered grease from burning.
Catch your temps on the way up. Using the Minion Method, start your fire with the top vent and all three bottom vents wide open. When it gets within 20* of target temp, close the lower vents 2/3 by either leaving each vent 1/3 open or close two all of the way. Always leave the top vent 100% open.
If you use lump, you need to pack it in tightly like your doing a 3D puzzle. The fewer air gaps the better. If you're using K blue briquettes, you will need to see a psychiatrist and send the WSM to me. Trust me, there are FAR better fuels for the WSM.
Don't take the lid off until you're getting close to start checking for doneness. If you're lookin', you ain't cookin'.