Im looking to to do a 240z or 280z build. The car will be used for auto crossing but want to keep it street legal. Im new to the Z scene and wanna know what are some good brands for when i purchase parts for the car

I want to put a mustang drivetrain into it (motor, tranny, rear end) will that fit?

What i should look for when I'm checking out a rolling chassis as far as rust, rot, etc

You might want to check out the Hybrid Z site, but if you are going to drop a V8 in, you'll probably need to stiffen up the body to accommodate the torque.