veritas wrote:

If the difference is the motor drive , I am now thinking that it may be a
better I idea
to start with the 503 CW body and accessories. Since the system is modular
I may then
adapt the 503 CW to become a 503 CWD if I want to in the future -- if I
can afford it .

Indeed.
The difference between CXi and CW is the CW's larger, non-vignetting mirror.
If you can live with the shorter mirror, a CXi (or any other model) turns
into a CXiD as easily as an CW into a CWD: they are all (500 C and upwards)
the same in this respect, all take the same digital backs, work the same
with digital backs.

But I was wondering if there are any books I could read to prepare me
before I buy the body ? Maybe a DVD I can watch ?

Ernst Wildi's "Hasselblad Manual" is a very good starter.
It comes in many editions, each containing a bit more info abourt newer
stuff.
You may be able to find one (or more) in your local library.