Welcome to the forum - lots of choices and lots to think about.
The oldest Airstreams have pipe frames that generally need a lot of rebuilding and so you might want to stay with the 1958
age range to avoid some of these issues.
Next: really old comes with a higher price tag. Also typically smaller = more dough.
How will you use the trailer? I am a musician as well and take mine to summer festivals. It's often tough to park a longer trailer on festival grounds. How many folks will be staying in the trailer? 19 feet is great for one or two but if you have a band - it's not enough space.
Airstreams prior to 1974 have no gray water storage tanks. Give this some serious consideration as many venues have NO services for trailers. You'll need to be able to store used water. You can retrofit a gray tank in during a restoration but you have to plan for this....
Start reading - you have a great adventure ahead