The only issue I think you are going to have is going to be with the AVCS.
You need the AVCS solenoids, AVCS hard lines that are on the front of the heads and AVCS cam gears.
Don't have one of these items and AVCS won't work.
Don't have the hardlines and you will burn up the front cam bearing as thats where it gets it's oil...oh and you'll have a rather large oil leak.
Don't have the solenoids and you will have a rather large oil leak.
As for controlling the AVCS, the only way you will be able to do it is with a standalone ECU....Hydra or AEM. The stock WRX ECU can not be flashed to control the AVCS. You will then have to manually wire from the standalone to the AVCS solenoids as you don't have the wiring to run it in your car now.