Re: 1st time impeller job
Your boat, order whichever parts you want. Mercury does make a kit, I believe, that has the impeller, bottom plate, gaskets, and misc. seals. So do several aftermarket companies.
Sierra is one rather prominent aftermarket supplier.
Search around for water pump kits for Mercruiser Alpha I gen II, you should get all kinds of places to purchase both Merc. and non Merc.
Right here on Iboats is a good place to start.