Sorry Dan, wasn't meant to be taken as sarcasm. I sense you're not confident so thought perhaps don't buy gauges etc and eat into your budget for the build and tune.
Best spend the money wisely on the build at this stage and concentrate on that.
Then get a Tactrix or similar just to get an ECU health check snapshot. I'm sure there's plenty of people around Sydney who've got a Tactrix to help you.
Commence reading and see if its possible for you to tackle your own checking regime as part of routine maintenance. Else, ensure continued contact with your chosen workshop and get them to monitor and manage it.
Just another option. No use having a heap of gauges with no idea what to look out for.
Also best to spend your tight budget getting the best rebuild you can afford.
Just my 5c. No sarcasm.

Ok Bruce, just thought there was a tongue in cheek sacarsim !!
I HAVE TO rely on the tuner and builder to do their work! Good thing is that it will be done by the one person!! Which equals a win win in my book.
The AFR gauge will be an ontop of the build cost as more of an insurance policy!!
So whats the thoughts on the innovative and the aem gauges !!
Not a Subaru...... Something MUCH better
NOPE..... That's shit too. Bus anyone??