Anything you buy new is a waste of money.
I don't think I would go that far. I buy both new and used and like both. Used is certainly an option, especially if you can wrench on your own bikes. I enjoy putting bikes together, but if you don't have the proper tools it is a pain in the behind.
If you are going to buy new I find something like the Giant Seek a good choice (okay, I have one, so, I'm biased). Other bikes like the Scott Sub 30 would be a good choice. You mentioned the Trek 520 and I also think that is a nice bike. Lots of choice, but leave enough money for racks, fenders and lights.
I actually like the mixte. I may buy one in the future to play with. You might think about throwing some new parts on it and giving it a new personality.