Welcome over Craig. I can answer a few of these questions for you. I personally have never had to buy windows parts on ebay, but to me they all look pretty much the same, but look out for parts by Morad Parts Company, because they'll have good cheap ****. Before you do that though what exactly is wrong with your window regulator. If you're just having issues rolling up you window check out the power window switch fix write up in the "How To" section, and try that before you spend any money.
97-03 had three engines: 3.1L V6(SE Trim), 3.8L V6 N/A(GT Trim), and 3.8L V6 Supercharged(GTP Trim). I'm not sure if they all did, but 97-99 Grand Prix SE's did have a 3.8 N/A engine in it. I don't really know about the trans though, but I think that the SE and GT had a seperate trans from the GTP, but they're pretty much the same exact thing, all automatic. Hope this helped you a little bit.
