Not sure exactly which ProBack you refer to; the only one I know of for the Contax is the DCS Pro 645C.
I own the camera, and the back. Within its limitations, it is a fabulous combo. Exceptional tonality, resolution, sharpness, and color rendition. Image quality has never been an issue with this back; rather, complaints have centered around its functional and ergonomic quirks. If you are shooting just about anything but sports or fast-moving stuff it should serve you well. I shoot mostly abstract still-life stuff, industrial / architectural, and portraiture, and I've not found its limitations to be a problem. I've even shot equestrian events with splendid results. (Between the Contax's sloooow AF---like a lot of MF gear of its vintage--and the back, it's kinda like the old days before good AF--you gotta anticipate and prefocus where stuff WILL BE happening.) The contax lenses are terrific across the board in my experience; the 120 macro is so sharp it could cut your corneas and may be the single best lens I've used in 35 years of shooting.
I almost always shoot at ISO 100, the lowest setting. Up to about 250 the noise is acceptable, at 320 bearable but obvious, and at 400 downright ugly. It writes slowly to the CF card, but as slow as the Contax's AF can be, it might take you that long anyway to re-compose and -focus, so no worries. The back can overheat in very hot weather also--it doesn't seem to like more than about 100ºF, and lets you know it by refusing to capture any more images until you get it a gin and tonic!
As for batteries--true, new ones aren't being made, but they are available out there pretty readily. I have around ten or twelve of them, and use them in strict rotation until they discharge fully, then in the charger they go. Digital Camera Battery, if they are still in business, makes/made a cable to adapt its external batteries for the C645. I imagine also that someone could rebuild an existing battery if you needed to, albeit for some $$.
A quick check on that auction site should give you an idea on prices for the particular pieces of Contax gear you're considering. Also look at KEH for what they're asking.
ProBacks on that auction site have not been selling briskly for 6-12 months (I track them regularly.) Of the three versions, the mamiya version seems to be the most frequently available, followed by the Hassy, and lastly the Contax. I suspect the C version would fetch a small price premium over the more popular H and M versions.
Kodak will support the back thru the end of the year; I'll probably send mine in for a checkup once my D300 gets here. After that, no one knows, so you do take a bit of a risk there. There's not much that can go wrong with them, but anything that does could be expensive even if someone can fix it. (Post-Kodak service might be something to inquire about with Kodak.)
As for the Contax--Kyocera has committed to ten years of parts / ?service from the date of discontinuance (?2005 ? 2006--can't recall). I have sent one of my Contax bodies to KEH for a CLA and minor repairs, with no problem. As many C645's as were sold (it was the top-selling MF camera for quite a while) I can't imagine that someone won't crop up to service them. (Ask KEH what they can do.) Also, you can get a backup body for not much, which is the strategy I followed. Big stuff like bodies, battery grips, and prism finders are easy to find; waist-level finders and many small accessories, and some of the lenses, are scarce and pricey.
I'd say, if you can get a good back for a decent price, buy it. You should do it if you can save real bucks over one of the other MF digital backs mentioned by others here. You should get at least a few good years out of the combo. This was my strategy when I bought mine (and I overpaid in my eagerness to have one, alas). Had it been available, at the price I paid, I'd have reached a bit further for one of the Aptus 17 refurbs that came available about six months after my purchase.
Oh, and ACR supports the ProBack files beautifully in PS, LR, and Bridge.
PM me if you need any other unsubstantiated opinions!
