Welcome to the Forums and the world of Vintage Airstream ownership!
If your coach has its original Peters and Russell (PAR) water pump, have you considered rebuilding? Unless things have changed tremendously in the last two years, it is still possible to purchase a comparatively inexpensive rebuild kit for your original pump (about 1/3 the cost of a new pump). Unlike today's typical RV water pumps, the PAR was designed with a very long life expectancy . . . and rebuilding and maintaining the pump can often extend its life indefinitely. Both of my coaches have PAR pumps, and while they can be a little nosier than the modern RV pumps . . . the noise is a small trade-off for their long-term durability (IMHO).
Good luck with your investigation!
P.S.: My recommendation for a new pump would be a new Peters and Russell . . . but they are somewhat more expensive than the competition. JABSCO/PAR Diaphragm Water Pumps