As you mentioned, we do have to find a replacement for Farrior soon. While the guy might still have something left in the tank, it's the last of his supply. I read somewhere that this is probably his last season, if he doesn't spontaneously retire mid-summer.
I don't, however, expect McClain to slide this far. He probably fits every single defensive scheme in the NFL, and I think he'll get compared favorably to Patrick Willis, when it comes to the impact that he can have on a defense. While Willis is a proven beast, I think McClain can have the same amount of impact for a team. Due to this, while it's not virtually impossible he slides to #18, I don't think it happens. If he does start dropping, though, someone will trade up for him, and since we're picking pretty high this year, I hope it's us.