I've been jonesing for a Redwood from these guys for years. I have grave doubts, however, about winter hardiness of this species here in Va... I don't think I could overwinter one without special (And increasingly expensive) cold greenhouse winter storage...
Don't bother.
Redwood can only made into bonsai when in optimal health. Even a slight decline in the condition of the tree will ruin the project.They only respond to bonsai techniques when they possess all their vigor.
If you are craving to own a redwood bonsai, there is one thing that I would recommend you try: buy a redwood bonsai that is small-ish in size (one-to-two feet tall, not too small, easy to change its location on your property), and it is already trained into bonsai. Get one that already has well-formed, compact foliage pads. The trunk is not a problem, they all have beautiful trunks. So, you don't have to put it through all the shocks of bonsai training, and the tree doesn't need to recover from those shocks.
All you need to do is maintain it. This way, the tree may be able to survive for a long time, in a low-energy state, without needing the extra engergy to recover from any drastic technique.
You see, the reason why these trees die is because they cannot handle too much shock. They like to be left alone. But they may be able to handle the pinching that is required to maintain them.
I know that for some people, it is above their pride to accept a tree already shaped by others, but this would be a small price to pay for the joy of having such a beautiful tree in their collection. Plus, the challenge to keeping them healthy would be enough to make one proud.
As to overwintering, all you have to do is to keep the temperature above freezing. A small, low-output electric heater in a garage can easily do that. They love humid summers, so that's not a problem for you.
The whole investment shouldn't cost you more than $370 plus shipping($120 for the small-sized material, plus another $250 for the training done in California, to develop pads).