These heads are so small and beefy they rarely warp. But you're doing a turbo build, you want them surfaced regardless. I can pick you up a set from the junkyard, there are 5 or 6 DOHCs in there, and plenty of intake manifolds.
Why 97-99 impreza? that's when they changed over to a phase2 engine. That intake manifold won't work, anyway, you need a DOHC manifold only on the DOHC heads. I actually have one, I think it's missing the EGR solenoid or something.