The rear dropout spacing may be a limiting factor if you're considering an 11 speed. What you definitely do NOT want to do is let anyone convince you to bend the frame in order to accommodate that. Do you know what rear spacing is? Hopefully it's 135mm which would let you do almost anything you want.
You're right that a 6-speed is most likely to be a freehub. If you want to go cassette, you're going to need new wheels. Technically you could change hubs but with the labor involved in rebuilding both wheels you might just as well buy a new wheelset. Then you'll still have the originals, which would enhance resale value.
That's actually a fairly special and valuable bike. If you are going to change a bunch of stuff, be sure to keep ALL the original parts. Don't turn a nice collectible into a Frankenbike, without the ability to turn it back. Actually, you haven't even ridden it yet so how do you know you want to change anything at all? Is it just because 1x is the current fad? There's nothing wrong with a triple.