The 645 will work with the pttl flashes the regular Pentax dslrs use, as well as triggering devices.
The problem with the original 645 lenses was that they sucked, the 67 and newer 645n were much better, so much so many used adapters and used them on hassleblads and contax 645 systems.
The 645d is getting some new lenses and the better 645n lenses are being re-released ( or just continued ) Mamyias $2000 free lenses won't hurt the 645D much, we can tell by the fact most of the used 67, 645 and 645n lenses dried up on KEH, BH and adorama as well as ebay, the prices are also creeping up on the new ones. For Pentax this was a long time coming and it's obvious they were waiting on tech prices to drop and the MF market to stabilize. I'm sure nat geo and many fashion mag shooters will be happy again, the 67 and 645n were location favs as unlike hassy and contax these were "FIELD" cameras that could take a beating, and in the $9600 usd price range it will dig into the high end Nikon and Canon systems for those who want high rez studio or location cameras ( IE not the sports shooters ). IQ should be the same as the mamyia 40 mp camera, but $20k ( with a lens ) vs $9600 and cheaper lenses ( plus many who have lenses already ).
in either case Pentax always considered MF as their flagship line and this time should be no different, now we'll wait and see if a 67D follows

OH camera Pron time

In the Photo of 645D lenses each row, left to right:
Top: 33-55 45-85, 55-110, 80-160 all FA's
Middle: 35, 45, 75, 120, 150, 200, 300/5.6 all FA's
Bottom: FA150-300, FA300/4, FA400, A600.
Largest MF lens line up