Very important. You must clean the brass before any die operation, or else you risk introducing sand into your dies, which will abrade your cases, depositing brass on the interior die surfaces, and the end result is not pretty or easy to fix.

Just keep the used brass clean.

Again, there are many different types and brands of case cleaners, the more expensive ones use water and stainless steel pins.

All you need to start is a vibratory tumbler, a media sifter and tumbling media.