I'll basically say it like I did for OMC.
Buy that boat only if:
1. It's REALLY "cheep",
2. The boat is in absolutely pristine condition (& no rot in the floor and/or transom),
3. You have a complete FWF ("Fire-wall-forward") Mercruiser engine, transom, drive, accy's already on hand and ready to install,
4. You have a buyer(and his money) for the Yamaha "stuff" you're going to remove to install the Mercruiser setup.
5. The money you got from the the Yamaha buyer equals what you paid for the boat.
6. You got a "swinging-deal" on the Mercruiser setup.
Then I would do it!
Cheers,
Rick