you need to have something like
www.domain.com to use owa, I would recommend sticking to a standard such as
remote.domain.co.za for everything. This makes life way easier.
Externally you would then always reference your server by this domain name.
to recap this means your DNS should look as follows:
remote.YOURdomain.co.za >> xxxx.dynDNS.com A record
your MX must then point to remote.YOURdomain.co.za
you might need an auto discover record.
If you are planning on learning more about Windows I would recommend you don't waste your time with 2003 as it is end of life.
Rather take a look at SBS 2011 as it configures exchange and works quite quickly, once this is all done you can jump under the hood and start exploring what is actually going on. As you get more and more conformable I would look at installing Server 2008 R2 or server 2012 R2 std and configuring the roles and features yourself.
The last thing I would always recommend for labs is if you can, go virtual. I usually deploy server 2012 std and Altaro Hyper-V backup (free edition). This allows me to create backups so that I don't have to re-install everything when I break it
Good luck and have fun!! Keep us posted if you run into any issues?
Nice to see that there is actually other people out there who lab as a hobby.