For my first one, I just picked one off the rack that had the nicest looking wood, didn't have any rust, and had all of the numbers matching, including the bayonet. For $100, you really can't go wrong. You'll want to make sure that you get the accessories, but especially the tool that lets you check the firing pin depth.
I was able to go back a couple of weeks later and really dig through the rack and find a much nicer, pre-WWII model, with a hex receiver. The finish on both the wood and steel are much better, but it shoots exactly the same (very well).
One thing I would watch for is how subtely they did the dot-matrix etching of the US import information. On mine they are lettered fairly small and near the end of the barrels, but I have seen others that are pretty sloppy and done right on top of the receiver.
Good luck. At the price range these are currently going for, you really can't go wrong as long as you can pick your own rifle out of a batch.