Do I need to have a website or my own blog to be successful with SFI?

Short and simple... no, you don't. But while they are not necessary, they can certainly help. 

Both types of sites have their advantages and disadvantages. 

Static Website 
Pros: relatively easy to setup and maintain 
Cons: harder to get listed in the search engines 
Primary uses: many use what is known as a static squeeze page. A squeeze page is designed to entice your site visitors into providing you with their contact details, thereby adding them to your mailing list. This list, most times, is loaded into your autoresponder account so that your list members receive timely, regular emails. You also have the option of mailing your list directly. Another use for static sites is to create a series of minisites, each one geared to a different aspect of SFI or focusing on a specific product on TripleClicks. 

Blog Site 
Pros: easy to get listed in the search engines, so long as you are providing unique, valuable content 
Cons: time consuming, unless you hire a freelance blog writer; then your main disadvantage is the added cost 

Primary uses: blogs are a great way to open up discussions on topics that are not only of interest to you, but also that you have knowledge to share; by providing useful content that is unique, then your blog can get picked up by the various search engines and you have the potential to build a relatively large base of followers who are interested in the content you are providing; once you have them engaged and have earned their trust, then they are naturally curious to know more about you and what you do