Something with 3/4 bedrooms anywhere near Central would be incredibly expensive. Your 24k estimate is ok for something small, one bedroom perhaps two. Nothing here is large; anything within 20 mins of Central typically very expensive (having said that, 20 minutes in a cab outside peak hours will take you a fair way - late evening I can get nearly home in 20 mins from Central and I live near Tuen Mun!).
Sharing is not common (although more common among people like you, so you might find someone). Unless you really want the company, I would not share for the sake of saving money because you probably won't. Try asking around your firm if you really want to share, you'll have more protections if you share with a colleague (much less likely they can run out without paying the rent if they work for the same organisation).
Most expats don't learn Cantonese - it's hard; not really necessary; the locals usually answer in English anyway if you try or don't even notice you are trying to speak to them in Cantonese. If you really want to learn Chinese, I'd learn some Mandarin. Quite a few people here speak it; more useful for business and if you travel to China you'll really need some Mandarin. But each to their own.
3. What items should you bring with you?
As little as possible (small flat; little storage). Unless you are a stickler for certain food or toiletry items, in which case bring a few packets (most stuff you can get here; some not - search for the various threads on "what do I miss most in HK" and you'll quickly figure out what's missing.
4. Suits for the office - who knows? That's a question for a work colleague. I've never seen a banker or lawyer in HK without a suit on (except while hiking!) but there may be some places where they don't require them. I'd be surprised though.
6. work culture for bankers? Banking is global and HK is a global centre. Go figure.
7. travel every other weekend? sure. airport is 25 mins from Central on the airport express. yes you will probably work long hours though. but lots of places are not that far away. once a month might be more sensible approximation.