A lot of my friends have Mako suits--I don't know if taxes change things for Austrailia but in the states they are considered a very good value. A custom suit can be VERY nice if you dive all the time and/or are a deep diver (especially in cold water) but it would cost 2 to 3 times what an off the rack suit costs. If you are able to dive in a 3.5mm shorty, your water probably isn't too cold and a perfectly fitted custom suit, while very comfortable probably isn't gonna be the best use of your money, especially for a first open cell suit. Unless you have an extraordinarily unique body an off the rack suit is probably going to fit you fine, provided you can go to a store with a large inventory where they'll let you try on some of the suits before you buy. Get a camo pattern that you think looks cool, better if it matches your local dive terrain (the fish won't care).
Some manufacturers will let you at least mix and match tops if you pay a little extra which can help you get a fit. I have a custom Oceaner and I love it but I certainly wouldn't appreciate it the way I do if I hadn't completely destroyed an Omer in 3 years of diving at least once a week, and in the process learned how to care for a suit both in and out of the water. Better to make mistakes with an off the rack suit, unless you have money to burn.