I would disagree with the reasons that Moko gave for doing spesh. Don't do specialist just for the scaling. Don't do specialist just because it'll make methods seem easier - if you feel you're going to struggle with methods more likely than not you'd struggle with specialist. Especially more so with distance ed. Those two things are just side benefits really.
I'm assuming that by correspondence means through Distance Education Centre Victoria.
http://www.distance.vic.edu.au/ I'm not sure if there's any other providers for doing spesh by correspondence, but anyway everything I'll be saying will be relevant to them.
I've done it via distance ed this year - I'm the only one at my school doing spesh. Doing it through distance ed is pretty awesome, or well that's been my experience anyway. In
some ways I prefer it over having an actual class and teacher. It might not be for everyone, you pretty much have to be quite independent when it comes to studying. You really do have to have a lot of motivation for yourself, you're not going to feel too much pressure to actually do the amount of work you should be doing. It helps to be interested in the content too.
If you're not, more likely than not you're probably going to struggle quite significantly (but there's still quite a few places you can get help if you are starting to fall behind).
They give you a book, one for Unit 3 and one for Unit 4 which contains the weekly work. That also has a set of notes and worked examples etc. and they explain things reasonably well. The textbook they use (Heinemann) is kind of rubbish though, I haven't really used that one that much this year. Only every now and then will they actually require you to do a question from the Heinemann textbook though.
The amount of work that they require to be sent in each week isn't too heavy, so you can pretty much dictate whatever pace you want to work at (I would recommend trying to stay a couple of weeks ahead of their pace, which is easy enough to do). The weekly work is basically a set of questions and a set of exam questions. The exam questions are optional, but you should do them anyway.
Most of the time you'd be doing your own study to understand the concepts and so on. Some weeks you need to dedicate a fair bit of time to spesh to get your head around things. They do ease you into things, and the first couple of weeks are really important for getting into a routine and figuring out your approach for the subject.
Your school should allocate you an extra block of spares to do spesh work in and that's a real bonus.
In terms of submitting work, I do it electronically. I scan in the work and upload it onto the distance ed portal (or you can email it directly to the teacher etc.). It gets marked digitally and emailed back pretty quickly (on a weekday it's within the same day usually, if not waiting another day). You can also post it there, but going over the internet is much much more efficient. The corrections are pretty in-depth.
In terms of getting help, the teachers at distance ed are pretty fast with getting back to you. I've never bothered but they give you the option of calling them up, going over some online whiteboard thing to discuss etc. I usually just go with getting help over email. There's also ATARNotes where I've gotten a lot of help from and the maths teachers at my school are usually pretty good with answering questions too.
This year anyway, there was 5 sacs throughout the year (2 in unit 3, 3 in unit 4). With the exception of the last two sacs, they're large. The first two sacs were 5 sessions. The third sac (in the early weeks of Term 3) is the two week analysis/application one. They were all fairly lengthy and reasonably challenging. The last two sacs are one session long.
The sacs get sent to your school and for the multi-session ones you give it back to your supervisor at school at the end of each session. Your school posts it back to DECV. They take quite a few weeks to mark them and post it back though.
In terms of resources, the internet is more than enough - especially this website. I've spent a fair bit of time searching old posts on this site when I'm trying to get my head around a concept. There's also quite a fair few good websites for maths.
You'll need to talk to your methods teacher throughout the year too, since fair bit of methods 3/4 is assumed knowledge for spesh (especially the calculus stuff, which pops up towards the end of term 1). It might turn out that your methods and distance ed spesh timelines don't really match up, and you might have to learn some methods 3/4 content before the rest of your methods class (only the basic stuff like chain rule, integration etc.)
So yes, if you really want to do it, it's definitely worth doing specialist. It's a pretty interesting subject and I wouldn't let the distance ed part of things stop you from doing it. You will have to ask yourself if you (seriously) think you'll be able to cope without a teacher in front of you - most likely than answer is yes, you'll be fine, distance ed isn't that bad at all.
I will say though, if you are apprehensive about distance ed, you would want to go through each reason why you are apprehensive about it and weigh up the pros and cons. You yourself will know what kind of student you are the best. You could probably ask your current maths teachers about how they think you'll go etc. You could probably give DECV themselves a ring and talk to the teachers there too.