Nobody in NZ does "Stages". It's a US term where they're all about getting a Cobb Accessport out and flashing the exact same tune on 500 cars with exactly the same parts on them.
I wouldn't bother doing anything if you're planning to get rid of it. By the time you "stage 2" it, and add some broadly pointless gauges, you're already spending... let's see... north of $3000. That's $3000 towards an STI, that will kill the crap out of a "Stage 2" WRX.
Aftermarket gauges are basically pointless. It's 2020, either let the ECU do it's job, or if you need to monitor the car better than the ECU can, get an aftermarket ECU and let that do its job. If something goes pearshaped, it's already pearshaped, knowing that your oil pressure just went through the floor almost certainly isn't going to stop your car going nogga nogga nogga....
If you have to do something to it, get some nice rims and decent tyres. Assuming you stay with a V8 (pretty certain it was V9 when it changed to 5x114.3) or earlier STI, it has the same PCD so you can change your rims over onto your new car when you buy it. Just make sure whatever you buy will clear Brembos. Likewise maybe some suspension goodies, just make sure they'll transfer.
You could start buying other parts that will work on your future vehicle, but arguably they either won't do much at all or will actually make the car worse without a tune.