I would start off by getting half your money back, because he sold you half of a nissan motor.
See if he has any 1JZGTE engines for sale instead.
If the motor is in good shape you don't need to rebuild it for 450hp. All you need is an upgraded fuel system (injectors, pump) and a single turbo swap. The stock twins will only support about 400hp. The engine itself is good for at least 500whp before you even need to worry about anything breaking.
If it's worn out, you can buy everything you need to rebuild it from driftmotion.com. If it were me, I would just buy another motor that's not worn out, since they are only about $1300. You can't rebuild one for that much.
Usually all you need to do to freshen one up that has some miles is change the timing belt, valve cover gaskets, maybe reseal the oil pan, and if the cam seals are leaking change them (if the cam seals are leaking the engine probably has over 100,000 miles on it and there's a good bet the valve stem seals are leaking too, so if you're doing cam seals, just go ahead and do the valve stem seals, you can do them with the head on with the right tools in about 6 hours).