I recommend that you watch the feature tour here to get a better idea of what powerstore gives you:
powerstore/
as far as paypal payment pro, you will just need to get an account with paypal and get the credentials required to use paypal payment pro.
Installing PowerStore is easy and straightforward and have a built in installer which will walk you through the steps and check to make sure you have the necessary setup to run PowerStore.
As far as changing the design, you should be able to make modification by modifying two main css files, content.css and common.css, but if you want to redesign it completely then you will need to use the main template.
