Want to buy 5DIII - question about light leak

I want to upgrade from a 7D to a 5DIII.  I did have my hands on one in a store here in Switzerland last week and have been in love ever since.

I recall reading about an occasional light leak problem on the first runs, and want to avoid that.  I also recall something about serial numbers to detect which ones had that problem.

My questions are:

1.  Is this a problem of the past and I should not worry about it when purchasing a new one.

2.  Is there something with the serial number or anything else I need to watch out for?

3. Any other additional advice?

Thanks.

--
Chris