1985 OMC 3.0 questions

Hey all. Im looking at another boat again... its an 85 stingray with the 3.0 and cobra outdrive. Found this on craigslist, came to find out its the girlfriends cousin...lol Anyway, he bought the boat in january of this year, and was told it had a new outdrive installed before purchase. (Doesn't look new to me with the beat up prop and skeg and such, but hey, he ran it for a while so who knows what he may have gotten into on the lake.) He said it was running great, then one day developed "a knock", and then he hasn't gotten it started since. Went to look at it last night and the battery was dead, thank goodness because the sump was full of water from the recent rains, and someone not watching could have shorted the system. I noticed a sheen of oil over the entire engine, but I wasn't super concerned with that, as it could be a bad grommet on the valve cover. Pulled the oil cap off, looking for milk, and it was good. Pulled the plugs to see down the holes best I could. 3 holes have a nice black on the tops, but not heavy as I can still make out details of shape on the pistons. The back one though, looked like it could have some corrosion on it. I need to check it again in better light. The plugs looked ok in all holes, just used. Been trying to find info on the engine, so I can analyze my options. Found where pulling a plug wire at a time can determine possible rod knock, and listening to the top end for lifter or pushrod issues. I Any other things to listen for to possibly determine what this "knock" is? Could be a bearing or u joint I know too. Also any info on the bottom end would be helpful, in case of rebuild being needed. For that, im curious what I could walk into a parts store and ask for, to get a better shake on prices. The kids behind the counter right now are bout half retarded, and only know what a computer tells them, and can't think outside the box at all.

Any advice or info would be much appreciated!