Just so happens I had the fresh water tank on my '86 31' Sovereign
replaced in the fall of 2014. The total for parts and labor was $751.82 and was split 60/40. Labor vs parts.
That was at JD Sanders in Alachua, FL (always quality work).
Since a lot of the labor is getting the cover on and off, I suspect you could do all three for about $1500. The tank itself was only about $150.
My 31', which is essentially identical to the one you are looking at, has turned out to be a delight to own and tow.
Look for softness in the floor, especially under the front and rear windows and just inside the door. One of the previous owners of my rig got behind on his preventative maintenance and had some chronic leaks so I had to replace about a third of my sub-floor. Not expensive but really tedious and time consuming.
If you do buy an Airstream, do yourself a favor and get it leak checked by someone with a SealTech machine first thing and then every other year. Costs about $200 but it will save you so much time and potential grief that it is money well spent. The big Achilles heal of any RV is the hull. Keep things dry and you will be a happy, happy camper.