scotty: Welcome to GTP! Have you tried searching for these questions? They may have been asked already.
1) For the best car to choose, look on the GT3 Writeups board to see if anybody has done a race report. You can put the name of the race series in the Search function and tell it to search for keywords in the thread titles.
2) For information about where to win certain cars, look in the Look here before you ask! thread that is permanently at the top of the GT3 board. That has prize car lists for both NTSC and PAL versions.
3) If you're talking about the Amateur GT World Championship, I ran it in a stock RX-7 LM and won handily. If you're talking about the GT All Stars, I ran it in a stock Raybrig NSX with NA tune 1 for some of the later races.