Unless it is close to free, I would pass.
If you want to check it out further, you need to drain all the water out, and check for the typical Laser problem sites - cracks in the mast step, softness on the sides of the cockpit (where you sit). That is in addition to visual inspection for cracks/holes.
Leaking through the toe plates are I guess another possibility, but from what you describe it's a lot of water. Shame - not beyond the wit of man to remove the plugs (one for the hull, the other for the cockpit) to let it stay dry!
Cracks in the mast step are a common problem; basically as you step the mast, it slides down with a crash; this puts stress on the mast step/hull connection. It is reinforced, but over the years the connection will part. Normally people cut an inspection hatch next to the mast step to gain access to the problem area, or you will see people who cut out an area around the mast step, couple of feet square, to fix it. There are many sites which will advise on this.