Well first off welcome to the forum
As far as buggying goes the most impressive buggies i've seen right now is made by the weldngod. check him out he frequents the site very often. He makes his own and you being in austrailia it would be cost prohibitive but for ideas and work of craftsmanship you gotta see his stuff.
Most buggies are made by a few companies. #1 on the list is peter lynn. I believe he's in the neighborhood maybe new zealand with dealers everywhere.
New are spendy. Used expect to pay 50-60% of new.
Most hold up very well depending on how you treat your equipment. I'm not a big fan of the construction I think the production models lack the sturdiest components when you compare what a custom buggy made by an individual can build.
The few things i've seen that could be improved are:
Seating the webbing on some of the seats have been known to break.
pivot point needs to be stronger some have broken parts where the steering pivots in the front.
Wheels are like farm grade and expensive to replace wish there where some more styles and tread designs.
A dream buggy some have been toting around are more back support and some sort of suspension system. Possibly a pivoting seat design would be cool.
Weldngod has made his own with more back support and a cargo area for carrying beers. hehe
Aluminum is lighter than the square tube frame that most production models have and are usually stronger and more heavy duty.
I guess its like anything if its going to be your #1 focus in kiting then don't hold back find a welder make some designs and build one i'm guessing in the neighborhood of 1500-2500$
If you just want to jump in and go and not worry about it by the production models. 600-900$ or cheaper.
there are racing ones long distance ones etc. again some are custom with bigger wheels for bumpier terrain + other features.
I hope I shed a little light on the subject. There is way more to discuss but you'll have to decide for yourself what it is your looking for in a buggy.

I love the wind and I love getting HIGH on air.