I use the 22.5 charcoal grate which is about 17 in diameter. You can't go wrong using the 18 or the smokey joe charcoal grate...it is all your preference and cooking needs. You can always get some expanded metal and roll it into a smaller ring to place it inside the basket you already have if you don't want to start from scratch.
I use a pizza pan attached to my basket as an ash catcher. Mine has no lip but I imagine one with a lip would be better as long as you do not restrict airflow.