If you want to keep it from breaking down and its in rough shape, you will probably want to look into: new hoses, belts, glowplugs, batteries, alternator, starter, return line kit, oil, oil filter, injectors, injection pump, and getting it timed. Maybe even the coolant, look up cavitation and running proper coolants with diesels.
If the owner used the wrong coolant then cavitation occurs, little airbubbles acting like jackhammers boring a hole from the water jacket to the cylinders. Not good.
My reasoning being this: most people out there don't do proper maintenance, when the glowplugs get tired then they crank harder on the starter, all that load can be ******* batteries/alt. If the hoses are original then it would be a good idea to swap - lower and upper rad, vacuum. It's not fun to suddenly lose power brakes just because the old vacuum hose is falling apart inside.
If the glowplugs aren't Motorcraft Beru (or Bosch but those aren't very good for the glowplug controller - better for manual glowplug control) there is a higher chance of them swelling up and breaking tips off inside the heads and mashing up valves and pistons until they find their way out or become one with the piston.
Test the lift pump, vacuum pump, alternator...
Electronic lift pumps are a great upgrade in that if you have air intrusion on the fuel system and/or run the engine dry on an empty tank it will make it much easier to start by purging air WITHOUT cranking the engine. The mechanical lift pump has to have the engine cranking to pump fuel. If you do have air intrusion you can just crank the engine several seconds before taking a break and trying to start it - until you fix the problems.
Check coolant for oil, check oil for coolant. Hopefully oil is in oil and coolant in coolant and not a mix
Could do a compression test to see how healthy the engine is doing, mid 400s or better is great. Less than that is getting tired, low 300s and lower is worn out.
The electrical system on these trucks tends to be crappy, get some electronic contact cleaner spray at the minimum. The passenger side has two fat wires that feed the glowplug relay but there is a connector between these and the battery. Often times corrosion builds up and fries this plug, most people just bypass this wire with one fatter wire. Can't remember gauges right now sorry.
What color is the exhaust? Gray just means it is cold, clear is good, black means too much fuel for too little air, blue means you are burning oil - bad rings, CDR pouring oil, turbo shaft leak, other oil source. These engines WILL run on oil, even their own. If that happens you have a runaway situation where turning the key off will not stop it. The only things you can do is try to ***** it out with CO2 fire extinguisher, blocking intake, or putting it in the highest gear you can and letting the clutch out while braking to try and stop it. Runaways are pretty rare, never had one myself but good to know about.
https://www.youtube.com/watch?v=rLzf3ItkbOE
Keep a couple cans of fix a flat in the truck somewhere, extra oil, extra coolant, maybe a couple belts, starter fluid (DISCONNECT GLOWPLUG CONTROLLER FIRST BEFORE USE - HOT GLOWPLUGS+ETHER=BAD), that should be a good start.
My old engine was dead tired and she was semi-reliable. Just didn't like the cold but otherwise would start. These engines are very reliable if they aren't super abused
