Those are very nice running engines with the phelon magneto. Typical Mercury weak points would be water intrusion in the gearcase and broken lower cowling mounts. Those tiny impellers are OK with annual servicing or inspection.
For me to love an old Mercury, there has to be something about its design or styling, that makes it unique among outboards, because otherwise, they are just a pain in the butt to fix, sometimes. I know those sixes run very nicely. But the ratio of uniqueness-to-difficulty in service on them is just not there, for me, personally.