See if the seller will let you perform a compression test before buying it, this will tell you what cylinders have issues. It may also show a bigger problem if the compression is really low.
The 305 will bolt right in but the freeze plugs, head gaskets and cam are unique to marine applications. Also as you said the marine parts need to be swapped over, carb, ignition, starter, alternator, ...
While I have never done an auto to marine swap I have built many auto and marine engines and replaced marine with marine.
1997 2400 SCR
5.7 Vortec / Bravo 2