It would be easier if you asked for next weeks lottery numbers.
Im doing the same at the minute with an old Merc 80 from 1979.
I went through it methodically, first job was carbs off and rebuilt, new fuel pump, new water pump and new plugs, I checked all breathers and balancing pipes replacing the perished ones, I cleaned all connections on the ignition side and starter motor to make sure I was getting a proper connection, lubed up and freed off all levers and linkages but fell at the last hurdle when my ignition barrel turned out to be fubar. In total Ive spent a week on this engine BEFORE thinking about turning the key.
Basically what Im saying is Ive tried to eliminate all chances of a problem. If I were you I would be tempted to go over all the basics first before trying to fault find.
As for your problem...It could be anything from a bad connection to a lazy starter to a bad starter solonoid. Chances are the carbs will be dirty too if its been laid up for a while.