Ok, so since you are serious, you will need to go through quite literally every last maintenance item before getting your family in harms way. Note, the motor is the easy part. Motor is a motor is a motor, just like your car engine from the '90's with just a few modification to make it a marine engine. But the drive and especially the hull are where your safety issues come in.
A replacement bell housing is easy - but the transom assembly needs all the rubber parts replaced as well as gimbal bearing and u-joints.
The drive is what it is. Needs to be removed from the boat, drained, and pressure tested first. You can do that yourself! There is a thread in the Adults Only section just on that. If you find leaky seals, they need replacing. The two drive cases then need to be split apart, the water pump removed and tossed in the can, everything inspected and lubricated, then new water pump and housing and reassembly.
HULL:
Where to start? Need to drill test holes in the low side of the internal transom to determine if there is much rot. Also drill the lower stern end of the stringers on each side of the motor. Holes are easy to fill and what you are looking for is dry wood or mushy old rotten mush.
Those are the things to start with. The bell housing is least of worries at this point.
You can get your eyes on the real service manuals at
www.boatinfo.com
You can also find several places to purchase the appropriate manuals should that become needful. Like
http://outdriveshop.com/mercruisermanuals.html
And this is the best place to get advice.
Happy rebuilding!
Rick