Personally I am not partial to Dutch Warmbloods for my own riding. However, were the horse to be sent out for all of its training to an experienced person in your area, Ruben O would probably throw a very talented horse.
Were it me breeding, and I breed and buy for temperament first, I would breed to the Donnerwetter stallion. If you look at the breeding of the dressage horses at WEG, a sizable portion were Donnerhall sons, or closely related to Donnerhall. I would breed both mares to the Donnerwetter, hope for a mare or two and breed back to the Rubenstein or Walt Disney lines.
Before you breed the mares, get them inspected for the Hanoverian studbook, and if they're accepted you can register the foals as Hanoverians, which will increase the value. I love Hanoverians, they are widely good tempered and also very talented.
If you aren't stuck on going Hanoverian, look for some Come Back II lines. He is a great sire, and although you can't get his semen in North America, he has some wonderful sons like Calecto V.