someone told me to buy a tank with nitrogen in it or just buy new tank??

The reason is that a bad tank can lose its charge in days or hours and will often frost around the neck.  If the tank is dry it is difficult to know if it is good or bad; if it has nitrogen in it and no frost it is most likely good and nitrogen with frost then it is bad.

IMHO a dry tank has $0 value when purchasing just because there is no way to tell if it is bad.  My last tank was bought dry, I paid $80 for the breeding kit with electric thaw unit and MVE 2000 tank that was 10 yrs old.