I agree only to an extent.Firstly -
Unless your school has shit computers and you have a shit computer at home, I wouldn't bother wasting your money - save it and use it for when you go to Uni. I had a Laptop and the only reason I found it helpful was because the other computer in our house is my parents work computer... so unless you don't have your own computer or easy access to one when you need it, I wouldn't waste your money.
I'm going to be the devils advocate and tell you a heap of reasons why you SHOULDN'T...
The whole thing with Extension 2 is showing you can think and create at an advanced level. You also need to be able tie it in with Advanced and Extension 1.
If you have a good Ext 2 teacher, you should be meeting with them reasonably often. While you might be able to start conceiving a concept of what you want to do now But you need to do research, and read similar styles of story and watch movies with the same types of concepts. If you try and start now your concept won't be refined enough, which your teacher should be helping you with.
If I were you I'd work on your writing skills. You said your creative writing skills weren't great. The way to fix that is to practice. Rather than using your time on one thing, work on the skill. Practice writing stories, essays, etc... improving your writing will only help with Ext 2.
Also make yourself widely read. Just read and read and read... and watch movies... movies that have won academy awards as well as movies that have interesting concepts. (I can give you a list of movies like that if you wish)
I would highly recommend NOT starting it yet, but just starting to think about what you want to do.
Oh and... read other peoples... as many as you can get your hands on... so you can do something that isn't generic and been done before.
If you're writing a short story, i think some preliminary writing off the top of your head can have a really positive effect. If you right for leisure, you will find imagery/ideas/characters you like, then you can retain them for your eventual major work.
I have to say I disagree with reading/watching movies that have won Oscars or Booker prizes. That's not really the point. You want to be reading/watching movies that are RELEVANT to what you want to do. For example, there is no point in watching an oscar-winning documentary on cats if your story is about dogs. Of course, in a perfect world, you would have the time to read as much as you want to have as much influence on your Ext 2 as possible, but if you want to start early, you want to start smart. What I mean is, start solidifying ideas, doing some preliminary research, finding an idea that you like, playing around with it and THEN finding research material thats relevant.
For example, i'm doing postmodern poetry about consumerism, and i got to that through firstly thinking about identity, an idea i stumbled across.
Identity --> Effects of society on peoples identity --> consumerism --> contemporary culture --> postmodernism --> poetry.
See how everything links together? That's how an idea is constructed, through research. Then when this idea is solidifying in your mind, you can start to write the proposal. That's what i did anyway, and i finished my MW months ahead of my classmates (I have another 4 to tend to, so i was strategic about it all )
Also, I haven't used a laptop and i'm fine. But thats mainly due to me not being able to afford one. It depends if you think it will be of worth to you. I would hate to have to carry it with me everywhere, but girls in my Ext2 class drool all over theirs, so its all up to personal preference.