Thanks for all of the replies and for the wise advice from Ky Grady. After further consideration I'm going to pass on the boat altogether. The seller's mechanic was supposed to call me back, which he never did, as well as the seller didn't want to go much lower than his asking price of $35k. Called up a bunch of local marine repair shops as well as Yamaha authorized dealers and prices start around $2k on up to over $5k, or more, depending on how bad the damage is. My local GW/Yamaha dealer Jacobsen's Marine in Edmonds said it would be a minimum of $2k and that's if nothing else inside the motor had any issues. They said that if the rest of the motor was corroded or rotted out then it could mean having to buy a brand new motor... no thanks. On top of that, some of the parts in the $600 kit were backordered and were going to be at least 4 weeks until I'd see the parts, then have a dealer install them after that. And imo it's just not worth the time and potential can of worms that I'd be opening up with buying the boat. Had it just been the $2k and it was a guaranteed fix that could be done in a week or two, then that would've been a different story.
To anyone else looking at the boat I mentioned, it's also for sale on Facebook and is mentioned in the For Sale section of this forum... the seller mentioned he bought the boat about 6-7 months ago but was selling because his brother is an alcoholic and lost his job and was the only reason why he was selling it. He mentioned he bought it from a dealer 6-7 months back and that he'd had the engine worked on before as well as thermostats and other things replaced since then. Which you'd think if that was true then the mechanic would've scoped the motor to look for or seen the dry exhaust corrosion issues long before now. I mentioned to him the dry exhaust corrosion issues that this vintage of Yamaha motors can have and then his story changed up some. He then said that he had his mechanic over early this morning looking at the motor, after I'd only talked to the seller for the first time yesterday morning, and the mechanic told him the exhaust corrosion was an issue. The mechanic claimed to be able to do the fix but was 2-3 weeks out, even though he made a house call within less than 24 hours? Either way, I've not heard back from the mechanic, the seller says he called him several times and asked him to give me a call, as well as I called him and left a voicemail yet no call back. As this point, there might not even be a mechanic? I try to give people the benefit of the doubt but in the back of my mind I'm thinking maybe the seller bought the boat from a dealer, didn't have it inspected, realized the problem with the exhaust 6-7 months later and is now trying to sell it because he doesn't have the money to fix the issues? Again, I'm not positive this is the case and I try to give someone the benefit of the doubt but I also haven't heard back from the seller after calling him a couple of hours ago and left a voicemail telling him about my concerns. Thanks again for all of the great info.
