You're in luck for rim choice. I just looked at the 2005 Trek catalog and the 520 has 32 hole rims so there is a wide choice of replacements.

Yes, your rim is a throw-away. The wheel can be replaced by relacing any suitable new rim to the current hub. You probably want a fairly heavy duty rim so avoid all the high-performance and Deep V aero rims. One rim I've had excellent durability from is the Mavic CXP33 which, despite it's name, is only 24 mm deep. It's moderately light and very rigid.

BTW, this type of failure is not uncommon with Trek's housebrand wheels. Two late 90's Kleins (built after Trek bought out Gary Klein) in my family both suffered the same rim failure you show but it took longer. One lasted about 12,000 miles and the other about 15,000.