If there are any leaks, there will be freeze/thaw cycle damage if the boat is someplace it gets below freezing for any period of time.
Concrete is a horrible ballast material for a couple of reasons, aside from the ones you've mentioned.
Concrete is not a very dense material. This means that it requires more space to provide the same amount of righting moment as lead or even iron. This means the boat has less interior volume and a lower righting moment, as the CG of the ballast is effectively higher. The boat will probably also have much more wetted surface area than it would if the boat had been made using lead as a ballast material. This means that a concrete ballasted boat will be slower and more tender than a similar design using lead or iron as a ballast material.
Second, it is difficult to properly repair once damage does occur. Lead is far easier to repair should you have problems.
Before committing yourself to this Rawson, I'd would recommend you read the Boat Inspection Trip Tips
thread I started, as it will help you determine whether this boat is even worth going forward on.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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Still—DON'T READ THAT POST AGAIN.