Cookiezi does not use anything atm as he is not playing, however I believe he used a cth-460 which is not for sale anymore (at least not in Sweden from which you and me originate from). Oh and a tablet will not make you any better, in fact, it will probably make you play about the same but you might improve faster. Cookiezi got good because he played a lot and challanged himself not because he had some superior gear. The only thing that will make a real differance is buying a mechanical keyboard to replace your current (I assume you got a rubberdome keyboard) keyboard.
Rest of posts will be something like this
"Yes, buy any tablet it will make you stream 5000000bpm and you will become cookiezi instantly".
osu! only rewards dedication, there are no real shortcuts to become better (at least none that will make you pro over a week or so).
I'd suggest buying a CTH-480S which is the small version and it will most likely suit your needs well. Another option would be to buy a tablet from Huion which I believe, do not quote me on it but I think peppy is supporting the tablets made by this company as they also manufacture the osu! tablet.