Hi there!!!

Welcome to the sport. 

Finishing an Ironman is all about training, and you'll do a lot of it, so you'll eventually need a fair amount of clothes just to train unless you enjoy doing a small load of laundry every day.  My advice is pick up a piece of gear every couple of weeks rather than buy a lot all at once.  As you train, you'll likely change sizes and you'll learn what you like and don't like. 

Swimming:  You'll probably want at least 2 swimsuits and you may experiment with different styles of goggles.  Even with 1 style, you may want clear or lightly tinted goggles for indoor or low-light swimming and heavily tinted goggles for swims in bright sunlight.  Lycra suits are common and may last 3-12 months.  Polyester is more durable, often going 12+ months.  I did have the butt seam on an old polyester suit fail during a swim...I probably flashed bystanders on every flip turn...

Biking:  You'll eventually end up with at least 4 pairs of bike shorts and 4 bike jerseys.  The really cheap bike shorts are pretty thin and wear out quickly (I held up a pair in the big box store, and could just about read a paper through the material)  I'll train mostly in bike shorts, but wear bike bibs (shorts with straps that go over the shoulders) for IM races.  Yea, they take a few seconds longer to put on (not an issue in a 12+ hour day) but you can read many IM race reports where people got a nice sunburn on their lower back where the volunteers didn't get sunscreen spread around well before they spent 6 hours riding in the middle of the day.  For IM races, I often wear my regular bike shoes rather than tri-shoes.  Again, in a 12+ hour day, a few seconds putting them on is inconsequential.  I usually try to train in bright colored jerseys to be more visible on the road.  For races, I've got a distinctive dark green/black jersey (you'll see it in my race reports) which makes it easier for my family to spot me.  Pedals are somewhat of a personal preference.  I like to train and race with Speedplay Zero pedals (the pedals look like lollipops).  The cleats are metal so withstand a fair amount of walking around without damage.  On shorter races, my tri-bike is set up with SPD (mountain bike style) pedals and the cloverleaf cleats (again, metal).  I like the Zero's because the "float" (amount of rotation before they disengage) can be high which may take pressure off the knees.  (Standard road pedals have no float, so the knee joint has to adapt to any misalignment during the pedal stroke.) 

Running:  Like biking, you'll eventually end up with 5-6 (or more) pairs of running shorts.  I got by with a lot of cheap tech-shirts, but it's worth getting high quality shorts.  As you run more, you'll be aware of which shorts you like and don't like and why.  Same with shirts.  Pay attention to chafing, it doesn't get better the longer you run!  Tech socks (usually inexpensive) rather than cotton.  Not sure how much the weather changes in the Gold Coast.  I was in Illinois with winter temps of -20C and summer temps of 30C, so had a fair amount of gear (shorts, tights, wind pants, short sleeves, long sleeves, shells) for the various conditions.  As you get to longer runs, you'll want a water bottle belt of some kind so you can stay hydrated. 

Running shoes:  Running shoes wear out!  They wear out far faster than it appears.  Best advice is to go to a running store (bring your old shoes) and have them do some gait analysis (so be dressed to run a bit) to see what shoes would be good for you.  (I was blessed with a neutral gait and could get by wearing about anything, but many runners aren't so lucky.)  I'd plan on 300 miles (500 km) before retiring shoes and would have 3 pairs in rotation.  Pair 1 (250-300 miles) for short runs (up to 6 miles).  Pair 2 (100-250 miles) for long runs as well as short runs.  Pair 3 (0-100 miles) for short runs.  Please don't wear a pair of brand new/unknown shoes on a long run!

Tech-wear care:  Heat and liquid fabric softeners are the enemies of tech wear.  Heat breaks down the material faster, and the liquid softener clogs up the microscopic pores in the threads that wick away sweat.  I'd wash my tech wear in a separate load, cold water, no softener and hang dry it.  As you can imagine, sweat can cause a significant "funk" and it doesn't always wash out easily.  I found that adding a cup of vinegar to the wash would reduce the residual "funk."