I know that every case is different, but I submitted a 189 visa application my self and it went through with no hitches. It seems that it is only worthwhile getting help if either a) forms and processes really aren't your thing or b) you have a complicated family/background/employment etc. The skills submission was a worrying wait, but the Visa process was a breeze.
And as for notaries - I submitted high resolution colour scans of everything without a single signature and there was no question asked whatsoever - I think this requirement harks back to dodgy photocopy days possibly. I couldn't find a hard and fast requirement anywhere of when to to have notarised copies, just advice of who can notarise.
Good luck.