Use of attachments with iExpense - are there issues with performance and storage?

We would like to use attachments in our iExpense application to have employees attach scanned copies of receipts at the time of submission.  We would like to find out what the implications are of doing so, specifically:

- are there issues with performance and storage (We have 5000 employees and use R11i)?

- is there a process for archiving the transactions for audit purposes?

- are there security issues with access to sensitive information on the attachments?  I don't think these attachments are brought into A/P when the invoice is created.

Any feedback on experiences in this area would be much appreciated.