The base for mortgages is 70% of the property value, although you can get up to 95% if you purchase mortgage insurance (this takes a few weeks to get set up).
Normal payments are 5% on signing the preliminary agreement, 5% more (plus stamp duty, which varies, but say 3%) on signing the formal agreement, and the rest on completion, which will be about 4 weeks later.
Getting a decent mortgage may be a bit harder if you are fresh off the boat and don't have a financial track record in HK. As a guide, I have just signed up for a 70% mortgage (of HK$3.5M) with HSBC where the effective rates are P-4 for the first year, P-3 for the second, and P-2.88 thereafter. Where P is HSBC's Prime lending rate (currently 7.75%).