Wow, awesome deal!

That is an incredible deal for a boat in the condition you've described. I recommend doing a "winterization" job on it so you can start fresh with the maintenance history. I'll post links below.

Use only genuine Mercury Premium Plus 2-stroke oil in this engine. I have exactly the same boat and engine as you do, and my boat's first owner had just started using generic 2-stroke marine oil in it when I got it. It was smoky just as you've described. I started using Premium Plus and once the old oil was consumed the smoking went away.

Before you replace the spark plugs (as part of the service linked below), you should decarbon the engine. Carbon buildup can also cause smoking, rough idle, etc. Get a couple of cans of Seafoam from any auto parts store and use it at 2-3X strength for a tank or two of fuel. This will clean out carbon deposits. (The really correct way to do this is with a standalone outboard tank and an even stronger Seafoam mixture, but your engine has so few hours this should work OK.) After decarboning and performing the service, use Mercury Quickleen in every tank of fuel. This will reduce carbon from building up in the first place, plus it will keep your injectors clean (another potential source of poor starting, warmup, etc.).

Ignore what the various and conflicting stickers and manuals say and run at least 89 octane fuel. (I personally run 91 octane because around here "premium" doesn't contain ethanol.) Yes, the engine has knock sensors so it can safely run on lower octane fuels, but experience has taught me that it runs BETTER on higher octane fuels. That's a $10,000 engine (no matter what you actually paid for it) and an extra nickel or dime per gallon to help it run better is well worth it.

If you take some time to get the engine to a known-good maintenance condition and keep it there, and feed it only good fuel and good oil and the proper additives, it will run flawlessly and you'll be very happy. Take good care of your equipment and it will take good care of you.

Link to 240EFI Maintenance:

http://articles.richardhartman.net/jetboat/mercury240efiwinterization.htm

Link to M2 Jetdrive Maintenance:

http://articles.richardhartman.net/jetboat/mercurym2winterization.htm

Link to Mercury 240EFI Service Manual in PDF format:

http://articles.richardhartman.net/jetboat/240EFI_MY2000/

Hope this helps!