1) No, you can't apply for a work visa without having a job offer first. The only way to be eligible to work in NZ before having a job offer is to have Residence, which you're already about to apply for, or if you're a young person with a WHV (an open visa which allows the holder to take any job, skilled or not), or the holder of some other kind of open visa, e.g. a partner-sponsored work visa.
2) Yes, it's possible to get an IRD number without having a job, but I very much doubt that it would give any advantage in job-hunting. All it means is that the Inland Revenue have got you in their computer ready to tax any income you may get, and as they'll do that for ANYONE, it doesn't say anything about your having a glowing work record or any such thing.
As I need a job offer to qualify for the skilled migrant levy
I don't understand what you mean here.