The drives were innovative, and if placed on the right hull (and a lot of time spent by the mfg. in their placement on the transom) they could be very fast. The fact was they could not perform well in as wide a variety of hull applications as a conventional drive.
Given the fact the Bravo was already in use in so many boats and enjoyed a decent reliability factor, the Blackhawk never took off.
There are a few places out there that can support them; I even see complete drives from time to time on Ebay, but what is your down-time worth if one fails?
Don't get me wrong, the P-29 was a sharp boat, and if it's something you have always considered as a 'must-have' dream boat, by all means pursue it. You only go around once in life. Resale may be tough down the line just because of the drive installation though, and I can almost guarantee you're going to want a bigger boat someday....