A '74 which has a grey water tank I might consider 3000 .
A '73 which has no grey tank I would not.
I'd say 2000 as did shipoke above, but if I was the seller into it for 3500 I probably would not take less than 2500.
Unless there are other circumstances & amenities that are very good to outstanding I would never go for anything older than a '74.
Gutting an Airstream completely is not to be taken lightly nor is it for the weak of heart (and patience). You must have some DIY (actually plenty) ability or you will pay, pay, pay a heavy and high price for help.
There are lots of 31 & 29 footers out there in the '74 & up age groups.
If you buy and gut and then give up, you will most likely not recoup half of your investment.
____ d'drummer ____