Michelle, I tend to be extremely conservative when it comes to driving training, so take my comments with that in mind.
After meeting Spanky, he is indeed an amazingly laid-back stallion, and you're doing great with him. In time, he will be wonderful in the HyperBike, but it is not designed for a green horse. For one, you need to have a solid whoa on him first, and that's a bit much to expect from a new driving horse. When I started Mingus with the EE, we still flew around our corral/workspace in the beginning, and it was nice to have a slightly heavier cart to help steady him.
A quality EE holds its value and will sell pretty quickly, so you could buy a good used cart to train in, get a few miles on Spanky, then sell it when you buy your HyperBike.