Leaks appear to be a quite common problem, unfortunately, and it can be very difficult to locate the root cause.
My observation is based on extensive research on a lot of forums plus first hand experience with my B50.
Possible sources are:
1) windows (both through hull windows and the opening overhead windows, if you have any of those)
2) the bedding of stanchions
3) the joining of hull and topsides - note that the water can travel inside from far forward and makenits way back to the aft cabin or a cockpit locker.
As far a s the rust on the keel bolt is concerned, inspect the entire keel thoroughly to make sure the boat has not been grounded.
I would insist on removing the seal between the hull and the keel to check that there are no hidden gaps, which could be sign of a grounding.
If you are suspicious of a grounding, ask the surveyor to check the stucture under the floorboards (he should look for small hairline cracks in the brown gelcoat used inside the hull.