Hey there Gaby.
1) Can't really say. I don't even have an ipad, so hard to recommend any. As a CSP user though, I really like it, but I don't know how it compares to PS or Procreate for apple devices.
2) The way I've seen it explained, it comes naturally as consequence of you practicing different styles you enjoy. So maybe you like how Dragon ball draws faces, how a particular comic artist draws heads, the way another artist renders hair, and through combining and practicing different things, you end up with something different. Some people consciously go at developing a style, and there are certainly many videos about how to go at it (just search "How to develop my own drawing style" and you'll see no less than 5 or more videos with this exact theme).
My recommendation though, don't worry about it just yet. I'd say focus on building strong fundamentals, experimenting, emulating others styles, especially those from works you enjoy, and when you are close to wrapping up the whole art course, or at least have made good progress, if you still feel you haven't developed a style and want to go at it, then consider about going through the style development process. Of course to each their own, there are some who are already developing styles from term 2, though it's usually because they already have a strong background in arts, and there are some who don't really care about developing a style at all.
3) I have a Kamvas Pro 16, and I couldn't imagine going smaller anymore. However, the tool has never been a deterrent for those aiming to do arts. I've seen pros who don't even like going digital, others that use a tablet without screen because that's what comfortable for them, and some mangakas like using a fairly simple tablet out of convienience becasue they can take it and draw everywhere (or share their work with the editor and what not).
What I'm trying to say, there is no right answer. Try to do what is comfortable and works for you. Maybe you move a lot, maybe you work from home and have good space for it. If you are happy and comfortable with your ipad, there is really no need to change. If you feel you need more space, consider your options, budget, work space and other factors to your situation and go for it.
The only real tip that has helped me which I would recommend is to not go for those "Expensive shiny tools" from the get go. As someone said "Buy the cheapest screwdriver you can find, once you wear it down, or realize it sucks, now you know why it sucks and what you value in the screwdriver, and can make a better decision when you buy a better one"
