Some of the older Trek 520s did not have touring-length chainstays. IIRC, this occurred for more than one model year and they were perhaps 43cm vs the 45-46cm CS length of the more desirable/useful vintages of 520s. Every time I ever found a decent lugged 520 on Craigslist it was one of these short CS models. Check Vintage Trek manuals for more information.

Also, older 520s don't have the 130/135mm OLD spacing of modern frames. A bike from before ~1992 with 7 speeds will probably have a 126mm OLD spacing. You have to either go with older (narrower) drivetrains, respace the dropouts, or put up with having to use a little force to change rear wheel. It's a good idea to have the derailleur hanger realigned if you're jamming a 130mm hub into a 126mm OLD frame, to avoid poor gear shifting.

Finally, on any used bike you're buying dozens of used parts that may or may not be safe to use - you don't know until you tear it down and inspect. Corrosion on a bike frame is sometimes hidden - you don't know the extent until the rust perforates a frame tube. Proper replacement of vintage bike parts is not quick or inexpensive. Often vintage parts cost more than an equivalent new component.

The new 520 would make life easier, and if you're spending that much, I suggest you consider a Surly LHT, as they're about 100 bucks cheaper and probably the better choice at even cost. You can also buy a LHT frameset for 400 and build your own bike from spare parts for less than the cost of a new bike.