The Smartcraft I had replaced the tach in the stock location. Run wire back to motor and plug into existing harness. Half hour job. No sensors as it pulls the info straight from the motor's ECU. That was on a 2006 Opti, not sure if that would apply to your motor.
A decent place to look and ask the question may be @ www.thehulltruth.com
That site appears to have a ton of outboard info from forum members there as well.
I just installed a Northstar 457 fishfinder that's hooked up to my mercruiser. You can configure it to display engine parameters / fishfinder / or split the screen and display both. The transducer has a built in paddle wheel for speed also.
You will need a can bus cable from the engine to the instrument panel. I would recommend getting the cable with resistors on both ends, this will save you a few buck in not having to purchase a junction box and plug-in resistor ($60). Plus then the cable will plug directly into your gateway and you won't have to find a place to mount the j-box. You will also need to get a smartcraft gateway, it's a "blackbox" with plug in short pigtial connectors on each end - one to the can bus cable the other to the instrument. It's plug and play, no configuration required.
I didn't have to purchase anything for the engine, the ECU sends the information it gets from all of it's existing sensors. There are several add-on sensors you can add if you like - steering angle, pitot speed, trim angle... Your engine may have them installed already??
Another cool feature is that it is also a fuel managment system, it sends real time fuel flow and level. it will also build a curve to show you the most efficient speed.
There are also chartplotters that are smartcraft ready if you don't need a fishfinder.