To completely change the look of your SBI! site it's best to do what I did and design your site in another program like Dreamweaver. Then you'll have full control. I wouldn't recommend starting out this way though because it can be overwhelming.
Even with the standard SBI! templates you can still do some customization (colors, background, etc.) You just have the most control when you use your own software.
You can make a shop. See
www.jugglenow.com This is an SBI! site with an integrated store/shop and payment options. Integrating PayPal is no different than a regular site. You just sign up with PayPal and paste the provided code into your site to generate the payment options/buttons, etc.
You pay $299 when you first buy it and then you renew in exactly one year.