No limit on concurrent connections, though your DB is too large for 2008 Express. It has a DB size limit of 4 GB. R2 ups that to 10GB, but remember that's a hard limit. Hit it and you won't be able to insert more data. Other than that, only testing will tell you if the resource limitations on Express will hinder your system or not.
Andrew, while there's no SQL Agent with express, that doesn't mean it can't be maintained. There are other ways to schedule maintenance tasks. Windows scheduler being the most obvious.
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: Discussions on DB performance with occasional diversions into recoverability
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