Use walnut for general cleaning since it's harder and scrubs better. Use corncob for polishing since it's softer and will hold polish better. If you order from Drillspot.com they will deliver a 40 pound bag to your door for about $26 total. You can't beat that anywhere, That's about two full 5 gallon buckets full plus a little extra. It comes in two grades or sizes. 12/20 is somewhat large, and will get stuck in the flash holes if you deprime prior to tumbling. 20/40 size is much finer and drops right through the flash holes.
I keep several Home Depot buckets for my media. #1 is what I use to clean dirty brass. Keep it for just that purpose. Adding used dryer sheets does indeed trap the dirt, and just throw it away when removing the brass and media from the tumbler. Makes the media last a lot longer. It's very important to keep dirt off the brass, whether you wash it or tumble it. Dirt will ruin a die very quickly. Bucket #2 is used for cleaning loaded ammo after loading, mostly to remove any lube that's on the brass. Tumble your ammo in a tumbler about 1/2 full of media for about 20 minutes, and the final ammo will be clean of all lube.
Buy a tumbler large enough so that you won't be spending a lot of time loading and unloading brass. Only you know how much you're going to reload and how often. Try to get a tumbler so you can run enough brass to load at least one session at a time. In other words if you're going to load 300 rounds of ammo at one sitting, then tumble at least that much brass at one time. Someone said, buy larger than you think you will need.