Hi Greg,

>I need a very good reason to start designing an object model first.

Well, IMHO, u will need it just if you want to have a "pure" OO
Analysis and Design approach, but let me tell you that then u
will have lot of troubles trying to have that Object model working
in practice. There are a lot of design and performance issues involved
here and different approches. Personally i preffer to concentrate
on the data model first, since you will not be using an OO Database.
I don't care too much for this, since ER diagrams are not quite
far from OO diagrams, indeed, i've seen some OO gurus doing
something like this when a relational database is used (Chapter
10 OO Design, Booch [basically you do it too Arnon :)]). This
will also let you take the best from your XCase tool, wich is
designed for relational databases, AFAIK.
From that u can construct u're base classes, maybe having in
mind 2 kind of components: GUI and Data processing. Also remember
that OO design is not intended to be a waterfall model, so
don't be so rigid in the design of your first classes. That is,
you can design a little, prototype and then refine your model.

I'll be very interested in knowing what approach are you
finally taking.

Ciao,

J. Luis