I did Specialist Maths 3/4 without the 1/2, and it was definitely doable. At the start of year 12, I scared myself about the subject way more than I should of, due to the subject's image of being impossibly hard, etc. This sounds really corny, but you just need to believe in yourself. After that I finally became focused enough to study for it properly. It was a lot of work, but enjoyable. What are your other reasons for wanting to do Spesh 3/4? Are you currently doing Methods 1/2 or 3/4?
I haven't studied over distance, so I don't have first-hand experience on what that would be like. You won't really be doing it alone, as your distance ed teacher should be contactable for you to ask any questions. I would also definitely take up the opportunity to get help from your teachers and see if they have any resources available for you, such as practice exams and practice SACs. Also, search the forums to see if anyone has talked about their experiences studying Spesh 3/4 through distance ed.