Trying to crimp an 8 point shell with a 6 point crimp will get you poor performance because the crimp will not hold long enough for the proper pressure to build. The second crimp closes the crimp and the final crimp compresses the crimp into the shell. The two different flavors of reloading presses are Single Stage and Progressive. With a single stage you are limited to one operation at a time. To go from deprime to belling you will have to pull one die out and put the other one in.
Progressives have all of the dies and the powder measure set up together.
For each pull of the handle the round will go thru each step. The true power of the progressive means that you can have a shell going thru each step at the same time. Once all of the stations are filled, you will turn out a completed round for each crank of the handle. The main factor in deciding which way to go is the scale of reloading. I have a progressive shothell reloader and a single stage for pistol and rifle.