Hi there,
The biggest pitfall for 189 visa is your working experience points.
So make sure that when you do your skill assessment you also ask for advice on employment experience. I think you need to pay additional fee for the advice letter from the skill assessment but usually co will ask for this later on. Other things like ielts and degree is pretty straightforward.
I see that you've done your research well. I've seen many people apply without an agent and has done so successfully. As long as you're prrepared to read ljne by line and do your research, i think you shud be fine....
Be complete with your employment document. Experienced eeemployment means you must be PAID for it. So make sure you have your tax or salary slip handy.