Uh oh. This might be rough.
There's a note here that says "The rack and pinion power steering gear used on these models should not be serviced or adjusted. If a malfunction or oil leak should occur, the complete steering gear should be replaced".
So... you're looking at replacement.
HERE'S THE GOOD NEWS - Rock Auto has some available for far cheaper than $600. If you have the standard rack, that's only $99 (plus $107 core = $206). If you have Speed Proportional Steering, that's listed for $185 (plus $70 core = $255). That saves you some pretty major coin right there.
Now the potentially bad news. This is not a fun job. About 4-5 hours is the standard for a mechanic. In my opinion, the hardest part is getting the old rack bushings out of there, then putting the new ones back in. My dad made a special tool for putting them back in and it is a lifesaver.
If it were me, I'd do it myself to save the $300. You could get the cheaper part and have them put it in for you, but they will probably get you with some "extras". You'll need new rack bushings, for an example. Also, when it's all done, it will need an alignment. I'm not sure if that was in your estimate or not. On the other hand.... you can pay them to do it and you won't even have to get dirty!
I'm not sure what you mean by the aluminum plenum. If you're referring to the actual intake, that stays on. The recommended instructions say to remove the throttle cable, wiper arms, cowl, air intakes, idle air control motor, wiper motor module, master cylinder... and a couple other smaller things. Like I said, it's not going to be fun, but it is doable!
If you need them, I can PM the step-by-step instructions to you. There are some basic diagrams in there with it. Let me know!