You have to ship a RB to you and parts are overpriced . The KA engines are no way able to handle high boost unless you can tune your ass off. They are iffy at even 8-10 psi. They have cast cranks and rods. The SR20 engines are the only 4 cylinder option from Nissan. I personally wouldn't touch a focus engine when you can slap on a Volvo head onto the old 2.3 and make shit tons of hp with a solid aftermarket.
If you want cheap hp, a 2j, 5.3 gm truck motor is the way to go. For 4 cylinders its a SR20, dodge 2.2/2.4 dohc, 4g63, honda K-series, chevy's ecotec, etc. There are so many ways to skin a cat.