mercury can rebuild for around $9,000. im sure local motor guys can do it for less. IMO let Merc do it.
Basic maint is minimal. change the oil, plugs, impellers once a year. Winterizing is minimal. drain the water, flush and fog the engine and treat the fuel. I would say 4 man hours at most. plus parts.
Drives should last for years if you dont tear them up on the bottom. And if your easy on the throttles.
They are generally rebuilt if the cases are in good shape.