1. If you don't know what SSL means, that means you shouldn't be using it at this point, it's for more advanced users and usually requires a dedicated server. It basically allows you to have secured page on your site to collect credit card info. You need to have that if you want to use PayPal Pro. You don't need that for the standard PayPal that comes with digistore or for 2co.

2. Anyone can purchase with paypal, even if they don't have a paypal account, they just enter their credit card info.

3. If you are already registered with paypal, then all you have to do is install the paypal plugin and then on settings page, settings tab of digistore you will find a place to enter your paypal email address. That's all you have to do. If you don't have a paypal account, you need to open one, go to paypal.com and register.