This has been asked several times that I have seen. The general answer is that it all depends on how it was taken care of. I have seen several people say get an oil analysis, have a detailed engine inspection (separate from a survey), and a host of other things. I think all of this information should be taken in and then put together for an overall picture.
I.E. an engine oil analysis is fine but much more valuable as an ongoing thing to see trends than a snapshot. If you don't know how long the oil was in and it comes back with no problems noted- how do you know it wasn't changed 20 hours ago?
Engine surveys are fine but again, short of pulling heads and getting in to measure tolerances tough to say how much is left in them.
I bought my boat (FL120) with 5,200 hours on it and the engine and tranny are possibly the best parts! ha ha They have worked with no problems at all. I took the Bob Smith class and did the things he said to do and have very high confidence with my motor.
Run the boat, take it up to WOT and it should get somewhere in the 2450-2600 range. Do that slowly and only after running it for awhile and with the brokers permission as if it is not in great shape you don't want to be on the hook for "breaking my boat"! Look for smoke and noises and such. Have someone that knows motors -diesel motors- with you. Experience is a big plus here.
Bob Smith said these motors really come into their own at around 5,000 hours. They should last over 15K before a rebuild. That was his words. He seemed pretty knowledgeable about them.
These are only my opinions and I'm sure others will have different ones. I think many people believe the FL120 is outdated and therefore obsolete. I find it super easy to work on and after taking the class, have no reservations about maintaining it at all.