sorry for the late response - there are various ways you could attempt this,
but it depends on what level of automation you really need. Also, I cannot
help thinking that some of the well-known contact management packages are
designed to do exactly this sort of thing, and re-inventing the wheel is
probably a rather expensive option in this case.

Personally, I would
a. focus on the /data/ needed to ensure that any merge operation can easily
extract exactly which records need to be used for which letters etc. for
your daily/weekly/ monthly merges. Without that, nothing will work well.
b. then identify how to select the relevant records - that depends on what
the data source is (Outlook, Access, etc.)

It's then a question of "how much automation do you want", e.g. do you want
to sit down each day and press a single button and have everything churned
out (or have an assintant do it), or do you literally want things churned
out automatically each day with no human intervention (which always sounds
like a very bad idea to me as it is very difficult to avoid potentially
costly mistakes).