Wotcha and welcome - The reason it has not been done and shared is because it is not economically viable for the amount of parts you would need to buy to make it work. That being said IMO you have approached it in the right way and got a complete donor car with all the bits required to get the transplant complete.
I agree with Savages feedback plus a couple of points.
At the very least you are looking at upping the power by 150 - 170 bhp so some of the 1.5 chassis parts will need upgrading. I have both a 2.0RX and an STI spent more time taking bits off than actually driving them. The 2.0RX as a snarling standard 150bhp ties itself in knots with spirited driving alone. The ARB's (particularly the rear) are small so look at bigger ones. The springs are soft so look at stiffer ones (not necessarily lower). The brakes give better feel in IMO than the STI which I have always found wooden (even the AP racing 6 pots I am running now) however they dont deal with the heat well and fade after a few enthusiastic corner attacks - look at braided hoses decent pads and vented discs flushing through with dot 4 fluid (highest boiling point).
While you have the car in bits look at changing the steering rack for a 15+ model item - gives a better (faster) ratio so you are not crossing your arms as much when hitting the twisties
The clutch is a push item.
The rear transmission diff on the RX is tiny compared to the STI so check that as you may need to use the Foz dif not just for the ratios but to handle the power or you may be looking at a WRX item.
Tools - Invest in a decent impact driver, wish I had got one years ago as I spend no where near as much time trying to work impossible angles in a tight space trying to get a breaker bar on a rusty bolt.
I would suggest replacing the rear diff support from an STI / WRX as it is alloy, lighter and stiffer bushes. That will help support.
Other than that keep a camera handy and set up a thread in the build section.