After taking the decision to establish a business and invest in Hong Kong, you must discuss with your accountant and possibly your lawyer what implication that has for your existing company formation. A branch of the existing entity in your home country may make your balance sheet look better, but has "reach across" capabilities for any legal or tax issues should the operation get into difficulties. Most businesses choose to have a "stand alone" limited company that is autonomous legally and responsible for its own tax affairs and regulatory obligations.
Similarly meeting an accountant and may be a lawyer to get the Hong Kong perspective on your proposed set-up should in most cases make it possible to agree what is the best option for your particular business model. You may want access the Mainland China market using your Hong Kong operation, which is possible, but there are things to consider.
Setting up a bank account is a straight forward process as long as you have the necessary documents relating to the Hong Kong Business. You need to decide if the company is going to use a "chop" or not, signatories and any limits or conditions imposed on them. But generally the whole thing can be put into place in one meeting.
As for getting an office, there a number of business centre's that can give you a virtual or serviced office at very short notice without having to make a long-term commitment.
If you're considering operating more than a shell company, any foreign national based in Hong Kong is going to need an employment visa. For new businesses the documentation required from either the overseas company or investor along with those for the Hong Kong operation is quite extensive.
Generally people don't appreciate the amount of time required to become operational, check what documents you have to bring with you. There is more information available now than there was in previous years, but don't under estimate what may be involved depending on your particular circumstances.