Re: "closed cooling" kit
Salmonee, It may not be too hard to install the closed cooling kit. 4 cylinder engines can use the original water pump impeller found on the MerCruiser alpha drives. V6 motors use either the standard impeller or a seperate belt driven thru hull water pickup. V8 motors use the thru hull water pickup almost exclusively. If you have the 4 or V6, you should be able to install the heat exchanger and do the replumbing fairly easily. If you need to install the thru hull water pickup and belt driven pump, the install will be harder.
If your boat has been used in salt water, it likely not worth installing the closed cooling. Also as was pointed out, there are full closed cooling systems and partial closed cooling systems. Full CC systems have antifreeze in the block and manifolds, but not the risers. Partial CC Systems have anifreeze in the block only.