Depends on condition and how it was used. It's a small-cubic-inch engine and needs to be propped correctly. Overpropping will kill 'em fast. The motor is propped correctly if it is capable of achieving its rated full-throttle rpm with a light load (one person in the boat).
The cooling system needs to be up-to-snuff (i.e. new impeller every year). Overheating just about guarantees a rebuild.
That being said, it's a 31-year-old motor and anything could have happened to it. Best thing to do is run it on a boat and see if it sounds good. If it pumps water, shifts, trims n' tilts, and runs out nicely, with no nasty knashing or thrashing, it's probably been well-maintained.
A compression check or (even better) leakdown test is pretty much mandatory unless you're buying it for Chump Change or as a parts motor.
I would run from any used Merc with more than 5psi difference on a compression test, regardless of what the books say about 10 percent difference, or whatever. And a variance between cylinders of more than 10psi is probably Bad News. The low-compression cylinder might be carboned-up rings, fixable with a carbon-cleaning additive, but could also be a scored cylinder or scuffed pistons/rings. No way to tell without a cylinder leakdown test.
A leakdown test will tell you exactly the health of the innards, but you might have to find a boat shop to do it, or pick up an inexpensive leakdown tester from eBay, Amazon, etc.
Just depends on how you're buying it, as an excellent motor for $$$ or close-to-scrap! A Salty Dog is of course gonna be worth a lot less than a motor that has seen fresh water all its life. I've seen fresh water motors run 50-60 years with little issues, and Salty Dogs burnt-out in 20 years or less.
There are still plenty of parts available, with pistons/rings/bearings/rods all interchangeable with a number of 4-cyl and 6-cyl Inline models.
So, I wouldn't necessarily be scared of one that needs a rebuild, as long as it doesn't appear too hammered. Merc used the same engine design up to 1990 or thereabouts.
If you expect to use this motor on a heavy boat, you'll have to prop it low and it's gonna run its little heart out. But on a smaller rig, they are fairly light and economical.
Note that the idle isn't as smooth as a Classic Four 40/45/50hp (the best fishing motor Merc ever built!), but is tolerable.
Anyway, hope that answers your question.......ed
