My initial reaction is NO, but should be fair and explain MY reasons why...
I will admit I am not a "lover" of 4 stroke outboards and wouldn't touch a used one unless there was a solid paper trail as to what maintenance had been done on it.
Unlike a 2 stroke that has ZERO moving parts in the head a 4 stroke has well over 100. After the initial break-in period (after first season of use) it is vital to the life of a 4 stroke that it be taken in to a dealer and have the first year service done. If done properly it will include adjusting the valves, changing the oil etc etc. This service will allow the motor to have a long life if it is followed by the recommended (read follow the Merc guidelines and treat them as the "minimum" that must be done).
My experience is this is seldom followed. Most often boat/motor packages are bought on "easy payment terms" and when a dealer wants upwards of 300 bucks to do the first year inspection (on a motor this size) the owner thinks "I only used it a few long weekends - it will be fine". This issue compounds until a number of years later the owner is getting rid of the motor.
(Often seen - "Motor for sale - runs PERFECT and in OUTSTANDING condition" - if that were true it wouldn't be for sale - at least be honest - it's for sale because it is either "NOT" perfect or it's unwanted and probably neglected)
A 4 stroke is not forgiving like a 2 stroke. A 2 stroke can sit outside, un-run for many years and with some minor prep be a fine running motor again. A 4 stroke left unattended quickly turns into a pile of scrap - the cost to restore often 2 to 3 times the cost of a new one.
Now, having said all that, if this 50 would well suit your needs it still may be worth looking at.
If this is a private sale, without a handful (read 7 years of service receipts) I would walk away unless the price was equal to the scrap value.
If a dealer has this I would still want to see service history plus I would want a minimum of 6 months "no hassle" warranty.
A brand new EFI 50 Merc can be had for about 6K (without looking to hard). If all the paperwork was in order I would still be hard pressed to drop more than about 2500 bucks on this motor. Without paperwork or a warranty, but at least a visual inspection and see it running, I would personally stop at parting with more than a grand from my wallet.
I will add though, a properly maintained 50 Merc 4S is a fine motor if treated properly.
You will certainly get varying opinions - mine is only one and it can be valued the same as the cost of the opinion
