The 120 hour course should be the CELTA or the Trinity CertTESOL. The Diploma would be 250+, although I'm not quite sure what these numbers are referring to. Generally, you will need to have the CELTA/Certificate to work as a language instructor. I'm assuming you have a Bachelor's degree? (Probably in something useful like "pottery" ) If you're open to or interested in working with children (primary and secondary school) you should really look into exactly what qualifications you would need to become a part of the NET scheme.
So, if you don't have a TESOL certificate yet, then you should probably be looking at spending some time upon your arrival at getting better qualified. It will pay off in the long run, even if it's difficult initially financially. I know English for Asia offers this training, and I imagine others might comment with other organizations. This is the PolyU program that is also recognized here (I only think of it because I know a colleague of mine has done this course and is doing well now teaching-wise... she's just a really great teacher, though).
Again, when you get here you might want to contact some of these tutor centers that offer private tuition for children/teens, as there's a lot of money in that. Kindergartens will also be very open to you, and then once you get your qualifications you can move on.
I came here because my partner was transferred with this job. I've been fortunate, as I've been much better off here doing what I do than I was before in London, where I was just barely making pennies. The teacher room in the language center I was at in London was a hot bed of resentment and negativity due to our low wages and poor management (and box ticking). I feel much more appreciated here in HK, and I've been able to find a niche for myself working for exactly the kind of organization and students I love working with. Do spend some time thinking about what group of students or work environment you would most appreciate in the long-term and start setting up goals for getting there. It's been hard sometimes in HK, though, because there's really only so many courses you can teach per semester, but then if you want to take a little holiday in summer you have to turn summer work down and then have no income.
Otherwise, I think living in HK is great. As with any place in the world, it has its upsides and its downsides, but what's most important above all is that it's different and that can give you a valuable perspective on life and your home. Be sure to explore Hong Kong's nature once you're here -- it's spectacular!
Is your teaching experience with a place like Berlitz where they just trained you themselves and take you for your "native English speaker" status?