Can l transfer an EJ20turbo WRX engine in my NA Impreza EJ18 car?

yes, but it will be expensive because of all the wiring work. You also need the entire car for the rest of the drivetrain parts.

I've heard that it costs around $10,000 in parts & labor to do the swap ''properly'' (so that it looks OEM & can pass california smog visual inspection)

You might be better off just buying a WRX, since this is a very big job. Also, consider installing a STi drivetrain, since most of the money is in labor, u might as well get the most bang for ur buck

WRX engine & drive train can be bought for pretty cheap now so it wo not cost that much. If u search around, there r some shops that will sell u a whole wiring harness for around $1500. Even with labor for the whole swap, l have seen some places doing this swap for a little over $5000. There r plenty of write ups on forums that will show u how to do this ur self but it will be a long project. Not something u can tackle for the first time & finish in a weekend. Check out rs25.com for lots of info on this. Many people with this very swap done so u can ask them lots of questions.

Yes. You will need to have a EJ turbo engine crossmember to do it, though. You will also need the ECU & extra ECU wiring, some find it easier to adapt the turbo wiring to the donor car.

Either way there will be some odds & ends to rewire, including some sensors on the motor itself, the spark system, & possibly things like the fuel pump & transmission sensors. Exhuast will need to be replaced, too.

It is a pretty involved job, but easily doable if someone with the expertise helps or does it for you. And l assumed u have AWD already, if it is a FWD car, then u can still do it, but l do not know how much fun having a one-wheeled wonder would be. :)