A few tips from my recent purchase experience putting together my Rollei HY6 kit.
DHW prices are reasonable, but they only take bank transfer for payment. Paypal will involve a hefty fee (6% IIRC).
The 180mm f/2.8 AFD-PQ lens is most certainly a bargain at $2000... I paid considerably more for a brand new one, so maybe I got a bad deal, but it is definitely worth the price.
Used Leaf AFI backs do show up on the auction site from time to time, as well as the B&S forums here and at getdpi.com. I'd suggest setting up a saved search on the auction site so you get an e-mail when one shows up. That's how I found my AFI-7 a couple months ago (but ended up purchasing on getdpi where the seller had duplicated the listing, saving us both quite a bit on fees).
All in all, you are definitely getting into a very much "alive" system. DHW are still selling brand new cameras, and while I haven't heard direct evidence that they are manufacturing new HY6 bodies, there are rumors that this is the case. At any rate, the system is a dream, and fulfills all that I want in an autofocus capable full 6x6 frame (with the new film back) camera system. I ended up with the 50mm f/2.8 AFD, 80mm f/2.8 AFD, and the 180mm mentioned above, and am able to cover the full range of landscapes, portraits, and the ever important toddler shots of my kids running around.
Good luck with putting together your system!