Hi Dylonchua,
First of all, your employer can actually sign a contract with you beforehand, but just adding a clause "subject your successful application of work permit".......I believe your employer must be a big one, to make Immigration to believe what's going on in your case.
Anyway, for the tax issues:
1. Everything you need about tax matters:
http://www.ird.gov.hk/eng/tax/ind.htm
2. Mandatory Provident Fund (MPF-in most cases) or approved pension scheme must be provided by all HK employers. For MPF, it's withdrawn and paid by your employer directly from your salary. Your employer and yourself should each contribute 5% of your salary on a monthly basis, capped at HKD1k. More info:
http://www.mpfahk.org/eindex.asp
3. Many things to consider in fact. But if you are enjoying some allowances (e.g. housing, sign-in bonus etc) from your employer, do ask the HR for more tax implications from those and would any of such tax be borne by the co.
Hope these help and good luck