air filter?

i have a question guys my friend has a '90 eclipse gs and he wants to put an aftermarket airfilter to it but not sure what would be best if to get the whole airflow system or if he can just get the air filter alone. what should he do?

I just put a K&N filter on my gs-t, and I hacked the dump tube.
But since he doesn't have a turbo, I would tell him to get a CAI. :)

Injen cold air intake.