Anthony,

... However it is not required to take the role of
the accounting pacakge at this time ...

As the client already has an existing accounting package
which they are used to this leaves you with a simpler,
easier to implement approach. Rather than duplicate
their existing ledger functionality, you could
simply prepare the journal entries they would need from
your application either on a transaction by transaction
basis or a summarised basis per transaction type per
accounting period (eg monthly). Depending on their
accounting package you may be able to interface to it
programatically or at least by text file export/import.
Depending on how far you wished to go, you could either
hard-code the journal entries for each transaction type
or else develop functionality within your application
where they could define a journal entry template for each
transaction type which you would then read & plug in the
account codes and amounts as the template indicated.

This approach would be fully consistent with any further
development in accounting functionality planned for
the future.

Cheers,
John