Roni, with enough growth of the prop, even if the hull is clean you can reach full throttle and have zero forward speed. On pretty much any boat.
If you have growth ("a good beard") on the hull and some on the prop, hitting 6 knots would be quite good actually. It can easily cut 1-2 knots off the speed of a boat.
Engine life and performance is a whole realm of "maybe". A good diesel, properly maintained and operated, should get 5,000-10,000 hours on it before it needs an overhaul. Not years--but hours of operation. If it is abused, the cylinders can get carbon deposits, the injectors can be ruined, and acid and sludge in the oil can ruin bearings while the cylinder walls and rings have rusted--all in the span of two or three years with near-zero use and zero maintenance.
Some maintenance is seasonal (before/after layup) while other things are base don hours of run time. If the boat doesn't have an hour meter ("Hobbs" meter) that tracks the hours the engine is actually running, they are not hard to add, and they give you a real count instead of guessing.
There's really no way to be sure, except to have a diesel mechanic take a look at it. He can pull the injectors, look inside the cylinders with a borescope (without remoing the head), test and if necessary overhaul the injectors, run an oil sample analysis, and give you specific answers about how healthy the engine is. That's probably going to cost you $300 or so, unless he hits the key and says "Gee, sounds perfect!" and tells you to skip the rest.
If you are unfamiliar with the engine, and diesels in general, it can pay to simply get a referall for a GOOD mechanic (which isn't always easy) and then have him come out for a couple of hours to inspect the engine AND to show you what has to be done to keep it running properly, how to bleed the fuel system, and so on. A much better investment than a tow or AAA membership.<G>