Yeah, art supplies are ones that last forever most of the time, so it's better to get too much than running out of something when you need it at that moment!
Those folders work great, and I love the portfolio A3 folders for putting full A3 paintings/work in, because it makes them look a lot more professional and it keeps them undamaged through the whole year.
Just make sure you don't spend the whole time making lists! I seemed to do that a lot, making a list then taking an hour break thinking I'd accomplished loads haha!
I think it's good to do personal projects, but I'm not sure whether there'd be enough time at A Level to do both, anything art-wise I did was just for the units over the last couple of years - I got sick of doing art by the end of it because of the huge amount of it, but then it feels odd not coming home and spending 5-6 hours doing art.
Maybe over the gaps between now and starting A Levels, and AS and A2 though? If you find yourself bored over the holidays, it can be worthwhile going to art galleries/museums, as they're amazing places to help with ideas and primary sources for coursework! (which you need a lot of) Just take loads of photos and leaflets, and then if you get stuck for ideas you can look through them and do sketches from them etc.
Yeah, years 12/13 are a lot more independent, but the classes are a lot smaller (ours went from 25 to 5 over AS-A2) so it means when your teacher looks through your work, they tend to look through
absolutely everything really in-depth which can be brilliant as they gave us lists of what to do next, and small bits that needing changing/adding to meet the mark schemes, as well as ideas to push the work more, and they might give you an idea of whereabouts you're working at now grade-wise (just don't take it to heart at the start of the year!). It might be completely different, though, depending on schools.
But, for a lot of the ideas they give you, they'll have the resources (laminating, odd fabrics, heat transfer etc) that could be quite expensive for you to go out and buy yourself, so use those instead of going out and getting it yourself that you might only use once.
Have you just finished GCSE then? If so, all the best for results this Thursday! If you loved art at GCSE then I think you will at A Level