Compression on each cylinder.
Rod end play on each cyl.
Vacuum and pressure test gearcase.
Check waterpump and thermostat operation.
Check over-heat buzzer for proper operation.
Check ignition for spark on all cylinders.
A good trained visual inspection.

These tasks take less than an hour to perform in labor, and a lube-change for the gearcase.
Lube costs about ten bucks and labor costs forty or fifty bucks.

Have the owner take you out for a ride and run at high speed for a few minutes, listen for buzzers, noises, etc.
If it runs and sounds okay, you'll probably be allright.

When you buy anything used; well you know how it goes.

Go back to the shop and have the carbs rebuilt; your fuel supply is your lubrication in a two-stroke outboard motor.
On that year Johnson or Evinrude motor, I would recommend having the oil-injection disconnected and mixing the gas and oil my-self.
The Variable Ratio Oiler has destroyed more than it's share of powerheads.
OMC should "Bite the Bullet" and change to a mechanical, Crankshaft driven, oil injection system.

By the way have a certified outboard tech. disconnect the VRO, tell him you still want to use the VRO pump for your fuel pump.
Good Luck and Hope this helps, Wayne

I always do them free of charge for first time boat owners, 30 bucks or so for anyone else. Don't really make much but it's good for business.

Always happy to help you with your questions on used boats and motors. LATER;WWW