First off, the internals need to be all forged. Crank, rods, pistons all must be forged, and must be made for this high of HP. I wouldn't bother with any "Kits" most likely they'd disentigrate above 4-500hp. I'd recommend something like some Pauter X-beams and I'd get Arias pistons which they already have on hand for a 350Z. As for a crank, I have no idea, depends on the strength of the stock crank and if anyone makes them aftermarket. May need to get a custom one. You MUST talk to these people about it and work with them to build the internals properly, these are just two choices out there (Excellent ones) but there are many others, and they will work with you to design the motor for its desired purpose. You can't just grab off the shelf parts for the most part.
Pistons will be somewhere $600-1200 probably, rods around $800-1200. Nobody said 850hp was cheap.
For 850hp I'd start looking into some major headwork also, and those turbo kits better have headers not manifolds. Two GT42R's ought to be plenty big.
I'd say the motor is going to be at least $5k, but thats even a dream of a figure. Once the motor is built, the whole drivetrain must be reinforced. Most likely the transmission won't last at that power nor will the rear end and things. Also, 550cc may well not be enough for 850hp.

-Andy