I have used a few different harnesses. I really like, a lot, the harness that Estate sells.
http://www.kee-port.com/harness.htm They are very well built. I would recommend the Deluxe for showing, I have several of these and love them. I also have a couple from Mini Express. I ordered one from them for my little tiny guy, the harness is very refined (I got the rolled version), and looks just beautiful on him. What I don't like about them compared to the Estate Harness, is the saddle. Estate harnesses have a real saddle with a gullet for spine relief. The Mini Express and many other show harnesses I have seen just have padding over the saddle. I think it is a little harder on the horse.
As for carts, a Graber is certainly adequate at all levels up to National. They are a lighter and less substantial cart, but look very nice. If price is not really an object, I recommend a Jerald. You can get one for a good price from Estate. They are on sale right now I believe. I have two Grabers and a Jerald. I prefer them for different circumstances. I have a small, lightweight Graber for my little guy. He would be overpowered by the Jerald. On the other hand, my Jerald is a very fine, substantial, and well-built cart that will last a lifetime with proper care. I use it on all of the rest of my driving horses.
I got my Graber cart from Debbie at Prime Design:
http://www.primedesigntack.com
I got my Jerald from Estate Horse Supply:
http://www.horsecarts.com
Hope that helps!