1. If you are feeling SUPER COMFORTABLE playing with a mouse, I'd advise you to become a mouse player. Sure, it takes slightly longer, and your sensitivity has to be super high, but you will also get more fame, and it's more cheap.
However, if you are feeling like you can't play mouse anymore, then it's time to get a tablet. One of the cheapest (yet best) options is probably the Wacom CTL-490 (Intous Draw) as it is cheap (for only £39 or something).
2. I don't know.
3. There aren't much. Firstly, you shake A LOT, very distinguishable with skins like M-NS, Default, my own personal skin and Cookiezi's skin. Secondly, there are different hand co-ordinations, and finally, tablets use absolute positioning instead of relative positioning, howerver, there are driver settings on some tablets to make it relative and it acts like a mouse.
Also optical mouse gives you good enough DPI to play any 7-10* NP.
4. If you are left-handed (like me), I'd advise getting a keypad, or using , and . to tap. It is very painful tapping with the 1 and 2 keys on my number pad. So, if you have the cash to spend, go do it.
4.1. If you think 'Hey, getting a mechanical keyboard will improve my osu! skills by a lot!' DON'T DO IT. Mechanical keyboards aren't such 'a waste of money' but pointless unless you have difficulty sledgehammering (but you won't need to do that until you can beat some 8* maps, and by then you'll be playing Relax). If you are a Mania player, go ahead. It's basically the tablet of osu!mania. So stick with a membrane keyboard for now. I play with one and I do fine. Somehow they are better that flat laptop ones. So if you want less delayed, more clicky and comfortable keys, mechanical is the way to go. But the change is so subtle, you won't notice.
Final Notes, For those of you who say "I need less than 1ms of delay!" I have to put up with 26ms of delay and it barely affects me (not at all in gameplay, a little bit in the song selection).