OP: the guys have been pretty harsh, but they raise some valid points. IMHO you should read between the lines and heed their advice.
Mechanic #6 definitely sounds dodgy, as mentioned you should only get a roadworthy cert if your car is indeed roadworthy. If he hands over RWCs that easily (aka a 'phoneworthy') then you have to wonder what kind of other questionable business practices they're involved in. Find a reputable mechanic and stick with them. The good ones are generally not the cheapest, and will often be the ones who will tell you what you don't want to hear rather than just saying "she'll be right", taking your money and sending you on your way. The first five you went to may have come up with different lists of problems but at least they didn't let an obviously illegal car pass a roadworthy. If you're on the Gold Coast then you have a few options, Gavin Wood for example has a pretty good reputation. If you're willing to go a bit further north, Elite Racing Developments at Yatala also has an extremely good reputation, and for any work I can't do myself is my mechanic of choice, despite them being an hour away.
If you want a turbo then check the SAU traders out, any one of them that deal with turbos should be able to give you plenty of advice, again ERD are a good place to start. As has also been mentioned though, you'll struggle to do much with $700, unless you can find a second-hand turbo, or maybe a Chinese knockoff of unknown quality (hint: don't).
As for your queries about car sound, check the Car Audio & Security section here on SAU. There are plenty of knowledgeable people around so as long as you're clear about what you are trying to achieve you should be laughing.
On a side note, the pic you posted isn't a GTR...