Trek made really high quality frames. I'm not well versed on the 620. but believe it is (still) one of the most sought after touring bikes. It sounds like the wheel upgrades were well targeted. As to whether or not it is worth $500, I'll leave that up to the experts on these. IIRC, I had read some comments previously about either the 620 or the 720 that many riders prized these over current touring bikes, priced at new well over $1000. Vintage quality components are every bit the equal and many time superior to modern in terms of durability.

As for downtube shifters, they can take a bit of getting used to. Where do you live, and how hilly is it? Where I commute (Chicago area) I rarely shift on the Trek 500 (mongrel components) that I ride. Usually just onto the big chainring (50/34) and ride the 50/19 the rest of the way, sometimes dropping to the 17 on the 6-speed freewheel. Since I have ridden D/T shifters all my life, they are no big deal to me.

Many people on touring bikes opted for bar-end shifters, so that would also be an option. Still friction, but might be more comfortable for you. From the pic you posted, it looks like it is really good shape.