We need lots of pictures and details in order to be able to help you. The rust is bad but I have seen worse and considering you got it for free, you're not out anything yet. Figure out how extensive the rust is though before you start pouring any money into it. If it's bad, you own a parts car.
I've owned several of these cars. If it hasn't been too badly abused, you could easily get 200,000 to 300,000 miles out of it.
nevertheless wrote:I also found out its not a B210 but a B310 sunny...
Just FYI...the 1979 B310 Sunny was badged as a Datsun 210 in N.A.