sounds like you have a lot of homework you need to do.
first 240sx I take it?

hit up the nissan tech section here.
do some googling.
get a better idea of what you are getting into.

4k isn't bad for a sr swapped s13, but that makes me wonder how well the swap was done. Have you driven it yet? like, really driven it. not just puttered around a parking lot.

Personally, I'm a cheap ass, so I'd talk the price down a lot.
With that kind of rust, I would bet that there is other parts of the chassis that is rusting too. So if that is the case, you basically need to look at buying this as if you are just buying a swap.
Might be better off to get a good deal on this, then hit up the nissan for sale thread, buy a new shell, swap all of the parts around and end up with a one good car.