See the "Individual Merge Letters" item on fellow MVP Graham Mayor's website
at:

http://www.gmayor.com/individual_merge_letters.htm

If you are using Word XP or later, the "Add-in to Merge Letters to Separate
Files" that I have written and that can be downloaded from that site will
allow you to create each letter as a separate file with a filename taken
from a field in the data source with a minimum of fuss.
Either way, that will allow you to create a personalised letter for each of
your clients.

Then you could use the method described in the article "Mail Merge to E-mail
with Attachments" at:

http://word.mvps.org/FAQs/MailMerge/MergeWithAttachments.htm

to use mailmerge to send an email to each client attached to which would be
the letter that was personalised for them.