A rock tumbler and walnut media will work for small amounts of brass. I prefer a vib. type tumbler with corncob (20/40) media. You don't have to put in additives but you can. If you don't think they are clean enough just let it run longer. You don't need to stay there watching it vibrate and can do other things while it cleans. Get a size that suits your needs.
I got a Dillon 750 and like it. Others will work well too, as already mentioned. Just get a size to match the amount you plan to clean at one time. When I got mine I was running it around the clock. I did several batches during the day and evening then a final one to run all night. I wanted to see if it would keep working even if it ran almost non-stop for the entire 3 day weekend. It did fine. Now I don't run it near that much. It helps to have something to sort the brass from the media. There are several ways to do this.