Another thought... if you're looking for maximum bang for your buck for TT usage, it might be worth splashing out on the front wheel, and ignoring the rear for now. The front is much more critical in terms of aero performance. E.g. the 50mm gigantex carbon 16h Derek has would build into a very nice front wheel with aero spokes. You could just run that with your normal rear and then upgrade the rear as and when you have the cash.
Another thought... if you're looking for maximum bang for your buck for TT usage, it might be worth splashing out on the front wheel, and ignoring the rear for now. The front is much more critical in terms of aero performance. E.g. the 50mm gigantex carbon 16h Derek has would build into a very nice front wheel with aero spokes. You could just run that with your normal rear and then upgrade the rear as and when you have the cash.