I've got a handful of early Treks (three for me, two for my wife). In fact, I was just fixing up a 1984 (I think) 620 for some off-road riding for my wife; I'd check the year, but it's several hundred miles away on a train at the moment. While I do love these bikes, the pre-1985 frames all have a weak link at the seat stay to seat tube junction. There just isn't enough metal holding those joints together, especially considering the large amount of flex in that rear triangle. I've broken several of them over the years. Fortunately, if you're more interested in the bike as a bike rather than as a vintage example of an American-built frame, it is easy for any frame builder to repair.

If the '83 520 fits you, snap it up and ride it. Just keep an eye on the "Trek" at the top of the seat stays. When it starts to crack, see your local frame builder.