You can buy individual parts. I would start with:
Front belts, either serpentine or V depending on year.
A couple of raw water impellers.
If you haven't upgraded to a SeaMax pump (Seaboard Marine's better raw water pump) then maybe one of those as a spare- you will need it some day.
A couple of Racor or whoever's primary filters you have.
A couple of on engine fuel filters.
A couple of oil filters.
Maybe a run solenoid- these fail sometimes and you can't start the engine with a bad one. Well you can if you know what you are doing.
If you need to buy a SeaMax pump then buy it all from Seaboard Marine. Brad is their parts guy and will make sure you get the right belts, etc.
But you also need the tools to change out the parts. I would go through each part and look at the bolts or whatever it takes to change it and make sure that you have those tools.
Unless you are going offshore out of reach of Tow Boat US or similar, that should do it.