I don't have an '84 520 but I have an '86 560. The 560 has more of a racing geometry but it is made of 501. It doesn't feel like my '84 760 which is made of 531c but its still a nice riding and well built frame. I have also had an '84 500 sport touring made of 501. It was also a nice frame.
The one thing to keep in mind, at least on the '84 500, and holds true for the 520 as well, is the wheels did not have stainless spokes and were built with Helicomatic hubs and either Matrix or Rigida rims. I replaced the wheels on both of my 500 series Treks from '84 & '85 when the rear hubs self- destructed. So even if you think you might like the Helicomatics, remember they came in a few different quality levels, and the 500 series Treks of that vintage, came with the cheapest ones.
I still think they are nice bikes, but much nicer with a nice set of wheels. If the price was right, I'd buy it anyway. I'm sure someone with direct experience with the 520 will chime in shortly.