What are potential security methods/mechanisms to ensure client data
and financial transactions are secure (for electronic business
systems)??
And propose a design that would provide this level of security??

What exactly you mean by "secure" here will have significant impact on
the answer to the question.  Security (according to the definition
used by CISSP, which is as good as any) is made up of three parts:
Confidentiality, Integrity, and Availability.  So - do you want client
data to be confidential between parties (in which case you're going to
want some encryption technology), to have guaranteed integrity (which
could have all sorts of different answers, depending on format, use,
transaction type, etc.) or have particular levels of availability?

In any of these cases, you're talking about more than $4.00's worth of
consultancy (which is why I'm not surprised that this question has
expired!).