with about $17,000 (trade-in value included) for down...and you want to keep your payment around $420. Assuming that you have excellent credit, your purchase price should be around $40-43k before tax and registration fee.
take a look here:
http://www.autotrader.com/cars-for-s...ortBy=yearDESC
i dont know how m3 coupes are priced in your regional market, but with your budget, I really believe that you can get at least an 2009-2010 m3 with under 40,000 miles. To keep your monthly around $420, you may need to finance it for at least 6 years.