Best money spent on helping improve the engine

1.  consistency, dedication and time.  Regardless of the tools and the toys, if you do this, 2 years from now you WILL be much faster than you are now.

2.  HR monitor

3.  power meter for the bike

Some thoughts:

1. Fresh Shoes - to avoid injury.
2. Bike Monitor including:
1. Cadence
2. Speed
3. Power (best, but expensive)
3. Trainer
4. pool (endless?)
5. treadmil?
6. And the best option - A nice private training facility with 100s of miles of unused but well maintained roads in a good climate. with multiple lakes/ rivers/ and ocean front property

first and foremost is a great work ethic and dedication. working at the sport for years will improve but as for equipmet swimming asides from goggles invest in a wetsuit unles your in a warm weather climate like hawaii.

for the bike go with the power meter, great investment but keep in mind its a tool and you have to learn to use it to get the best out of it. also the power meter is expensive. then an aero helmet, can spend anywhere between $100+ on it. then last go with the wheels, your best bet is to buy a used set of aero wheels like i am. if not id suggest get a good front wheel and a back disc cover or if you have money to burn go as fancy as youd like

for running just a good pair of clothes and shoes will do just fine.

last but what i argue as most important nutrition and recovery. make sure to buy and eat the right food, get good in, get good out. make sure you consume a recovery drink that has protein in it, i go for soy milk since im lactose intolerant.

those are just my two cents of stuff to get.

I think that the best bang for my train-the-engine buck has come from buying a good quality bike trainer (a KK). It allows me to get in structured, hard, and accurately measured bike workouts year round, regardless of the weather, amount of light, time of day etc., and in less time than it would take outside. 

I see that you're in Gold Coast, so an indoor trainer might not be a priority. I actually prefer to use the trainer in good and bad weathers, as I think I can get better training if I don't have to worry about traffic, road maintenance, my own safety, etc.

There's one thing that I would place higher than that. It's having the good sense to listen to your body, to avoid getting hurt so that you can continue training. Dedication can be good, but moderation counts for a lot too. I have a spotty record in this department, and I pay the price in injuries. But I'm not sure that there's a price that can be put on sensible choices.

For me in the last two years, AG MOP to AG 1st in last race, its been this:

1. Bought a KK Fluid trainer and have done Jorge's cycling program 3 times so far.  Bike went from 20-20.5mph to 23-23.5 mph on the same sprint race courses.
2. Several pairs running shoes.  Started training for HM after tri season, its taken my runs & fitness to another level.

I've been doing Tri's since 2004 and have basic training "stuff". I currently have a low-end Timex HRM/watch, aero helmet, used 40mm wheels and what was a high-end tri-bike in 2006.  No GPS, no Powermeter, no deep dish/disk wheels, although I'm working on those! In 2005 I began training with HR and did the two field LT tests (Bike/Run) several times as training has progressed.  Best thing I did back then as it taught me to slow down and learn the proper RPE.

Best money spent?

1.  A good coach

2.  Gifts for your significant other because of the extra time you are going to dedicate to training!

Running - Garmin Watch with HR

Cycle - Trainer for winter

swim - wetsuit

My quick list of "engine improvement items" to buy "earlier in your training so you can build on them" (OP):

1.  Running.  Agree on a good training watch - stopwatch at a minimum, but would splurge on a gps watch with HR monitor.  Good shoes, but that is pretty obvious.

2.  Bike.  Agree with many on a buying a good, quiet trainer (KK or CycleOps fluid) for more steady year-round bike training.  That with a decent tri or road bike will get you 90% of the way.

3.  Swim.  Coaching if needed to learn proper swim form up-front and then access to a decent lap pool.

I agree with the above.  I started 2 years ago and these were the BIGGEST help for me in terms of making my training worthwhile (and effective).

For the bike, above buying a trainer (since you live where it's warm), I would rank having a bike that you enjoy riding.  My first year, I trained on the bike a lot because I loved riding it.  I never missed a planned workout.  My second season?  Not so much.  I didn't like my new bike so I tended to blow off those workouts more than others. 

If you already have a bike you love riding, then yes.... a good quality trainer should be next on your list.

I'd say work on getting to your "race weight" and being consistent with your workouts first. I don't think you need to invest any money for these things.

Once you get there, maybe reward yourself with some of the toys to keep you motivated. 

Common training limiters that I have found in myself and observed in others has little to do with the technology we use.  Bikes, helmets, wheel sets, power meters, GPS, and, yes, even coaching are the icing on the cake, so to speak.  I'd bet that what's holding most of us back from training right and attaining our best results is more mental and we can't buy ourselves out of that one. Learn that one and save yourself a ton of money.

But, you did ask what to spend money on to improve the engine.   I think you need to determine what's holding you back.  So, ask yourself first these questions:  What are your biggest limiters to effective training and What is slowing you down in racing?

Then get the tools you need to address the biggest issue(s) that pertains to you.  Don't waste your money on someone else's fix.