I would just look for the normal stuff like, tires, belts, hoses, the condition/color of the tranny fluid. These are are pretty durable and known to last 200,000 miles easily with some maintenance. My coworker purchased a 2002 Echo about 2 years ago with 121,000 miles on it, he now has 164,000 and the thing runs great. The only things he has done to the car are oil changes, tires and front brakes. These little cars are surprisingly durable.
Thanks dorman, I just wish I could find used echos easily. On craigslist, there's usually 3 or so...with salvaged titles...or 160K miles, etc. I could get a Gen 7 or 8 Corolla, but they both seem to have a few more problems than the echo. Don't want to spend more than $3500 or so.