It sounds a good buy especially if it has a new head. The stock head is weak. I have seen several cars with 603 engine have had a new head as early as 120000 miles. Be sure the engine sounds normal. A bent rod or a loose timing chain gives noises. The 603 engine sometimes gives valve tapping noise during startup and it should be very quiet after warmup (comparing to 617 engine). The 603 engine does not give out smoke from the oil filler hole when you open cap while idling. But if you see a little smoke, it is okay. Look thru the oil filler hole while engine is off; the cam should be smooth and shiny. You can tell the degree of blow-by by looking at the the air filler, around the oil filler hole, and the cross-over joint (intake manifold) on top of the valve cover. Other major areas to check are the coolant color, tranny fuild color, A/C system.
603 is a good engine. But like buying any other used car, it is better to find someone know about the car to check it out.