No it's not necessary and if you're comfortable with researching and forms there's no reason you can't do it yourself.
I would only recommend an agent to someone if they really can't put any time or effort into it or if they have complicated circumstances (child custody, criminal record, medical issues, age deadlines, etc).
We did our permanent visa application ourself. It was the 136 PR when we did it but now it's called the 175 visa. I believe the sponsored 176 is pretty much identical except your first step is to get state sponsorship, which should be too hard to do.
I would read through the link I gave you on the IELTS question, then read through Booklet 6 for more information:
http://www.immi.gov.au/allforms/booklets/1119.pdfAfter that check out this link and scroll down to the bottom where it has the associated forms:
http://www.immi.gov.au/allforms/booklets/books6.htmHave a look through them and see how easy/difficult it is.
I managed to do our application all by myself with lots of reading and questions on the British Expats site:
http://britishexpats.com/forum/forumdisplay.php?f=32On a further note using an agent will not "ensure success". If you don't meet the criteria the application will fail whether you have an agent or not. In the same way if you meet all the criteria then having an agent will make no difference. The only thing the agents do is give knowledgeable advice and check over your application. You're pretty much still the one to fill in all the forms and do the legwork. However, a good agent should be able to tell you whether you meet the criteria or not and most reputable agents only go that last step with the ones who are going to get a visa (i.e. if you are going to fail they will tell you upfront).
I did a search on the British Expats site about agents, here are some of the results:
http://britishexpats.com/forum/search.p ... id=1043263Not all of them are relevant but if you read the titles you should be able to pick out the ones that deal with using an agent or doing it yourself.