You should purchase two reloaders since switching them from 12 gauge to 20 gauge and readjusting them for proper crimps is not an easy task. You'll need a scale to weigh powder charges. You'll need shot and powder bars and powder cups for the load you will be loading. A reloading manual for load recipes is helpful although load recipes are available on the internet from the powder manufacturers. A funnel to pour shot and powder into the bottles is very useful. That will pretty much get you started and if you really get into it ther's always additional things that come in handy.
