How much does the seller want for the engine? How much do you want one of these RD models? What is the overall physical/cosmetic condition of the engine? Will the seller allow you to pull the plugs and sneak a peek inside the cylinders with a light? Will the seller allow you to crack open the lower g/c lube screw to inspect the lube?
So many variables, hard to advise about this purchase. This is a gamble, so you need to feel comfortable buying it, only to find major problems that will keep the engine from running without a major investment of time and money.