ATRPilot - Do some more reading. You are not going to join as a DESO. You will join as a Cadet.
If you're girlfriend has no particular skills, she will find it VERY difficult to get a job as she will also need a work visa. No skills - no visa.
Yes you need to pay tax on your loan. Yes you pay double tax. Yes it will disappear into thin air.
20k /month in Hong Kong is peanuts to live on. Good luck. I have lived here a looong time and 20 grand vanishes. Especially when you are first starting out.
Buy an apartment with 880k? Not sure where you're going to do that. Top heavy interest in the bank? Clearly you have not done your homework and without that, you are venturing into very precarious territory.
Like I said, do some more reading and don't get blinded by the BS.