I need to be able to check the datatype of the selection criteria
entered by the user matches that of the field being queried upon, and
if the datatype  does not match, if it can be coerced in to the
datatype of the field.

What Im trying to do is duplicate the querybuilder that comes bundled
with SQL enterprise manager using VB6.

Also does anybody know if the diagram pane in the enterprise manager
query builder is available as a reusable component, or if there are
any OCX's available out there that mimic the diagram pane?

Anybody have any thoughts on any of this?

There is a couple of OCX for sale that do this for you. It mght be easier to
just buy one.