I think it matters what your plan for it is. On
ebay right now it's probably worth nearly $20k after power-washing off the oil off the bottom. If you're going to paint it, that's going to cost a lot and be a lot of down time.
If you aren't going to do the work yourself, it needs some thousands of dollars worth of seals, gaskets, cleaning, etc. Figure it needs all new brakes and hoses for another few kilo-dollars.
If you're going to buy it, fix the leaks and drive it as is, $14-15k seems like a good price. It's damned original, radio, emissions, etc. Mine is pretty original, but it doesn't have the vacuum hose looms like that one does.
That paint could be hiding some ugly bondo or other bad body work around the rear sill.
So the bottom line is that the seller is probably going to want more than it's worth, but if you really want it, you may have to pay the seller. Make a spreadsheet with costs before you buy it.