Might be getting a used tank that is completely empty now.....?

Okay so two of my mates is selling a 4ft tank complete with stand and accessories and for the price they are asking its a BARGAIN which I'm going to see tomorrow to have a good look at it and get all dimensions.... Anyways I know its been used but completely empty now and has been dry for a period of time which I'm not sure of.... But how will I know if the seams will hold up(if I get it) when I fill it up? How long approx should a tank be 'dry' for before the seams dry out? I've heard somewhere that if a tank has been dried out for too long the tank won't hold. Usually I take my mate who knows fish and tanks but he may not be able to come along with me

Can anyone offer any advice? Or what I'm looking for when I go and have a look? Any help would be appreciated