As a general rule most diesels are solid workhorses and last for years but a few tips that may help you.
Before starting the engine check the engine it's cold to the touch, then check the oil level, especially check to see that it's not milky or any water on the dipstick. Check the water level and if it has anti freeze, especially check for little globules of water floating on the surface.
Start one engine only, check the moment it fires that all cylinders fire immediately and no missing, check the water is passing out of the exhaust.
Shut the engine down and do the same sequence for the other engine.
If you can afford it send a sample of oil from each engine for analysis.
Take the boat out for a run and monitor the temp gauge and both engines should run evenly at about 85/90' C. check the oil pressure and it should be between 40/60 psi when the engine's warm.
The engine will smoke a little at first then as it warms the smoke will disappear to a slight wisp under load.
When you come back to the dock, CAREFULLY remove the oil filler cap to see if there are lot of fumes 'puffing' as the engine runs on tickover (blowby).
The Ford 6 cylinder is a great workhorse and like any engine as long as they are serviced regular they're fine.
Some tips especially for Ford diesels, always keep anti freeze in the coolant as it keeps the liner seals soft and a good seal.
If it starts to 'make diesel' there are 2 suspects, 1 is the diaphragm in the lift pump, 2, the injectors are under the rocker cover and sometimes the excess fuel return pipe which joins all the injectors has a brazed join at each injector, as this brazing is brass it 'work hardens' and over time may crack due to vibration. Both jobs are an easy fix.
The engine is still made today in Turkey so spares aren't a problem, I'm sure there are many excellent dealers in the US but there is a guy called Mike Bellamy who runs his family business at Lancing Marine in the UK who used to race them and is considered by the trade to be the UK top expert on those Ford engines.
I hope you find this helpful and I wish you all the best with your new boat.