If you really are interested in a digital back, I recommend one of the current systems. I know it is a drag to change systems, but in the grand scheme of things, you will likely get a better deal buying a packaged camera, lens and back from one of the back makers "preferred" companies. You also get the ability to get new parts, easier repair and lenses that have been made with the higher resolution sensors in mind. While certain Contax lenses will have no problems with the higher megapixel backs (like the 120mm), some of the others might (some of the wide angles).
Overall, my advice would be that if you want to get a good deal, you should be flexible about what system you want to use. Unfortunately, the investment you have in Contax gear will be very small in comparison to the cost of the back, so it is best to find a good deal on a back and then find the lenses to use on it.
And as a Sinar user, I will say that while the adapter system is a great thing, it is not something that you would want to change on a regular basis. It is a little harrowing to be working with a screwdriver millimeters away from a 645 sized sensor. One false move and you have lost 10,000 dollars. They are designed well, so there is little chance of making a mistake, but it is still not something you would ever want to do in the field, or even on a regular basis. The real utility of it is that you can change yourself fairly easily if you decide to change systems.
