5200 hrs and original is probably a reasonable indication it's been taken care of. Having said that, it is probably 40 years old so bargain hard with the seller. I hate to say it, but it's a buyer's market.
If you do buy it, and there are no records, plan to change the oil and filter, the on engine fuel filters (at least), set the valves, check the integrity of the fuel injection system and particularly the return lines (all at the same time as the valve work), replace the raw water impeller, clean or replace the heat exchangers, buy a spare lift pump, change the oil in the Simms pump, check all the belts and burp the coolant bleeder. The transmission will love you for an oil change too! Look carefully at the exhaust and hoses...
I'll think of other things. The only difficult bit is the valves, otherwise you can do it all yourself.
Lehmans smoke and put fuel in the water at startup, this is normal but might be a bargaining chip. Excess fuel means it's time to repair or replace the injectors.
Have fun!