I replaced the steering rack on my wife's 2003 GT last year. I purchased a used steering rack at Ed Morad's (moradpartscompany.com), so there was no core charge. Based on what I read online, I chose to purchase a used/checked-out as my impression was these lasted as long or longer than the non-GM aftermarket versions. Shipping was $30, or so.
Get a write-up for a detailed, step-by-step removal/installation procedure. You can find several on this website. I also downloaded the GM Service Manual procedure from my local library's web site.
Finally, I'm an electrical engineer, but fairly mechanically-inclined. It took me about 5 hours to remove, install, and test the new/used rack. If I had to do it again, I'm certain it would take about half the time. It's not technically challenging, just takes some jostling units in/out and time to get access to hard-to-reach areas, specifically, spline connecting steering column to rack.
Oh yeah, you can verify whether your vehicle has Magna-steer installed by looking for the "NV7" RPO code located on the inside of you trunk lid. Good luck.