I'm doing Specialist 3/4 via distance this year (not many schools in my location offering that subject LOL). It's going pretty good so far.
I think you definitely will need determination to do well in the subject, and the fact that you get little personal contact with a teacher makes it crucial to be able to learn the textbook on your own. Personally I began studying the course in November of my Yr11 and (roughly) covered up to integral calculus (not always using the VCE textbook, mind you, and a range of concepts I had covered to some depth in the past) by the beginning of term 1, which was a pretty good advantage.
Don't worry - I was in your situation at the end of year 11, not having done Spec 1/2. Although I did go to some tutoring/coaching classes for SM 1/2 in the latter part of the year just to 'see' if I was up to the level, I found it very doable. This year, I did attend a coaching class for SM, but largely for the purpose of being able to 'see' the competition (e.g. where I stood in relation to other students), as I am a stay-at-home student completely with Distance Ed. In my humble opinion, I would have been able to attain a similar level of performance had I not gone to tutoring. If you have some doubts about your abilities, I would advise you to start on some of the topics earlier on to get a 'feel' for the subject, even if it's an older textbook (borrow from the library, or use some resources such as Khan Academy). If you can understand the concepts without too much trouble, you should be OK. If not, get a tutor (I personally went to DHMC and I would recommend that)
Regarding teachers at DECV for Specialist, they have consistently provided me due feedback on my work usually within 1 week of submission (you send ONE assignment (or SAC) per week, and it gets marked). My teacher is very friendly and helpful.
Hint: apparently one of the teachers at Distance came from JMSS

All the best with your decision