Sitting for a year is not going to cause a good engine to go bad so if he says it needs a rebuild now, it needed it a year ago. Without tearing down, its a gamble on what you are getting. You can look for obvious cracks, drain the oil and look for signs of water, loook for signs of big oil leaks, remove plugs and see how clean the motor was burning-black, not good, light tan, good. Remove valve covers and look for broken springs and sludge. With plugs removed try turning the motor by hand. If it does not move, then it is suspect because a motor sitting for a year should not "freeze up". Ask if it has been bored previously. I never want a 283 bored more than 40 Thou over for daily driving cause they get hard to coool.