I have a 2015 350ss with 8.2 seacores. I am the second owner, bought with about 150 hours and now just over 200. I am in the northeast as well, and the boat was here since new. I can give you some good things to look for. My boat was in amazing cosmetic condition, zero rust visible on the engines or anywhere else, had binders of marina maintenance receipts and a good survey and sea trial that give me plenty of confidence. But the 2020 season I had a lot of down time. If I were you I would make sure of the following things:
1) I lost a month in the 2020 season diagnosing, then waiting for parts, for what turned out to be 2 clogged "cool fuel" modules. It is CRITICAL that you flush and drain these each season during winterization, and it's likely good that you try to flush each time you use the boat. Mine were completely blocked with salt at 150 hours to the point that the engines would vapor lock after 15 minutes of running. They would not restart for at least an hour, making for some aggravating days. If you think the previous owner didn't flush and skimped on winterization I would look at these modules carefully and make sure seawater is flowing freely thru them. The passages are tight at the inlet and outlet and easily clogged. If not you'll have vapor lock at the worst possible time. If you pay a mechanic I would budget $4-4500 for a replacement of both. They are about $1500 each by the time they get to your door. At the time I dealt with this the supply chain was spotty and the modules were hard to get. Another sign that this issue is looming is if you see any history of fuel pump or fuel pressure regulator work. The symptom is sudden drop of fuel pressure after lower speed running where the fuel is moving slowly. The temptation is to change out regulators and pumps trying to fix the fuel pressure issues. But it's the modules. This is all a result of no flushing and not draining the modules completely for winter.
2) Shortly after this I had to do both raw water pump assemblies. No issues with flow, but the bearing at the pulley started to leak on one, then the other. Early symptom is rust streaks on the back side of the pulleys. These won't leak unless the engine is running, and in my case only at higher engine speeds. In other words it would be easy to clean salt and rust stains from the bilges and engines if the boat is on the hard and you'd never know unless you check BEHIND the raw water pump pulley. This is another $2500 ordeal to do both.
Access on both these items is a total nightmare on the 350ss. a 310 might be even worse. At least for someone my size. And that brings me to the item in your post, which I am in the middle of dealing with now. Risers and manifolds on these ECT (cat) engines. I am at 6 seasons and just over 200 hours. Historically, rule of thumb is 5 years in salt water. I saw a tiny bit of rust around the starboard riser to manifold gasket on the port engine. So my mechanic advised to disassemble and inspect. I found the seawater ports at the manifold and riser junction plugged about halfway. The bottom one was worse. Plenty of visible rust within the seawater passages of the risers and manifolds, and corrosion and evidence of salt in much of the "dry" portion of the risers, telling me I may have caught this just in time. But they look fine on the outside. So I've been researching my options, and other than replacing with factory parts, there is no reasonably priced alternatives. Good news is the cats slide right out and are re-used.
I considered CMI stainless replacements, which I liked for several reasons. WAY lighter, much less bulk, and likely an easier install as a result. But it's about 10 grand to buy them for 2 engines, and then you need to have Whipple reprogram the computers to deal with the cat removal and 02 sensors. That didn't scare me, but the costs did. This is a $12K ordeal in parts, plus labor.
I also looked at doing the reprogram and then using the manifold and stainless riser system from a non ECT engine, so that going forward I would be looking at periodic replacements of just the cast iron manifolds. With everything needed, it's $7K in parts, plus the reprogram, plus labor. Still way too much for me.
Ultimately I decided to stay all OEM. I was able to get all the parts yesterday for under $4000 for both motors. So in reality, the parts themselves for these engines are not really priced out of line with the same factory parts for a non-ECT 8.2. What worries me is the work. The ECT manifolds are so big and heavy, and access is so tight, I am dreading this job. But I think it's money well spent knowing I can get another 5-6 seasons, and who knows, maybe more, if I am diligent with flushing, which I don't believe the previous owner was.
Sorry for babbling, but to answer your question I think based on what you're seeing and what I've seen, these newer manifolds, even on seacore engines, last the same 5-6 seasons in salt water as all the other older systems do. So if one has a leak, get all of it replaced.
I will create a separate post to see if anyone here has been thru an actual replacement on the newest ECT motors like mine with the single cat. The previous gen ones had 3 cats per manifold and they came apart in multiple sections rather than just one, likely making the work much easier. But the newer single cat design has just been out long enough now that we will be seeing a need for replacement for the first time.
I do flush, the engines, after every use. I can't speak to the freeze plugs, but I imagine they'd have them. I guess the first thing would be to determine exactly where/what is leaking, then you can make a more educated decision. If you have other specific questions, I be more than happy to jump in.