You can compare the frame geometry on the Vintage Trek website. The 520 was most definitely a touring bike, but at that time Trek also made a 620 and 720 that were more geared toward heavy loaded touring. The 720 had longer stays. They all had caliper brakes, though; cantilever brakes were just becoming an option around that time. My '82 720 has caliper brakes, and I continually toy with the idea of having cantilever studs put on.

Don't I see a pair of eyelets on the fork? Regardless, with P-clips you can attach racks pretty well, I wouldn't worry about that. You'll have enough room for fenders with 27" wheels, and a little more if you have 700c wheels.

All in all, though... $300 is not an amazingly good deal. I'd distrbute the cash among three pockets in my jeans and go see the bike. Your observations about eyelets, geometry, and brakes are all good talking points; add to that the 27" wheels... and see if you can get it for $200.

At any rate you can certainly tour on it and be proud of it. Though the 520 was not Trek's top-of-the-line touring bike at the time, it was the one that really caught on, and on which the current 520's reputation rests. The 620 and 720, though idolized by some, were soon discontinued and even today have plenty of detractors.