Would I come here with a job offer of 18k? Yes. What about 18k + the serviced apartment? Yes.
If they are paying for your housing, then it's good. The whole thing is an experience, and if it's something you want to do, and this 18k job is getting you here, then why not. But I mean they should be able to sponsor your visa and get approval before you come, I think. I heard from another company that the rule of thumb is 18k and not 20k. Yet, when I first came here, I had companies try to pay me 12k and were ascertain they could get a visa. I wouldn't worry about the visa, it's merely a formality, if the company is newer then it's more paperwork. Also, people argue oh it takes 4-6 weeks to get a visa, well you will know this argument isn't really a problem, yes it might be true in some or most situations, but you will know what I mean after you live here.
Can you live on 18k? Well, if you are making 18k only, I would say it's not great. But once you are here, you can get a feel for what your pay should be, and then you can find another job with better pay. Plus, once you are in HK, your job market opens much more! And don't be afraid to leave your first job after 3-6 months if the conditions and pay are matching up to your wishes. Transfer sponsorship is very possible. I've seen companies threaten they will cancel your visa and you must leave, don't believe the nonsense, you can switch, you fill out a transfer form and there you go.
But I can tell you, when I first lived here, I would be happy with 18k, but now after being here, I seriously won't consider anything near 18k. If you want to settle down, and get enough space, you need more money than 18k.
And as far as living in TST....I think there are affordable places to be found everywhere. I feel like even though the general rental prices are going up, it seems that I am exposed to more of the "real prices" the longer I live here, and the longer I live here, the cheaper the flats seem to get, compared to the "expat prices". It's a game, I can't explain it, but it takes awhile to know the ropes and once you do, you can find better housing at better rates. All I can say is, well don't trust the first 50 real estate agents, and thereafter, keep your guard up.
Best of luck! Hope you can make the move!