I can't speak for the '85 520, but I own an '84 620 and a 720. The 620 is simillar to a 520 but with slightly better components and steel tubing. The 720 was Trek's ultimate (paved road) touring bike, with even better components and steel.

There is big difference in how 620 and 720 handle due to frame design and components. The 720 has a longer wheel base and is much more comfortable to ride especially when loaded. The 720 also has lower gears that make it a better choice for loaded touring.

In regards to fenders, I could not mount a front fender (in the conventional manner) on the 620 because the clearance between the top of the tire and the fork crown was too small. There is no such problem however with the 720. They have slightly different forks but the same 27" x 1-1/4" wheels.

Another issue when riding vintage bikes is maintenance. If the bike your thinking of purchasing still has the original components, you may find that it's impossible to find replacement parts at the local bike shop. Or even a mechanic that knows how to work on the old components. You can of course upgrade to modern components but this has a high cost and require spreading the rear chain stays to accept a 130mm hub.

My 2 cents: Get the sport/touring 520 if you plan to do a lot of fast commuting and an occasional loaded tour. Or get a dedicated touring bike for loaded long distance extended touring. Don't get either for off road touring.