Welcome to the forum! I have a 2006 340 that I've had for 8 years now. had a 260 before this one. I'm a big Sea Ray fan and kind of bummed they got out of the "big boat" game. This was my first twin engine boat and first that had a closed cooling system. Everything else pulled raw water to cool. I run in salt water and I didn't have to bother with heat exchangers before.
Heat exchangers were original to the boat when I bought it so 9 years old at the time. I spent a good amount of time chasing heating issues, loss of coolant, etc. until I replaced the heat exchangers (engine, transmission, and oil cooler on each engine).
Typically when I buy a used gas boat, I always replace the manifolds (and potentially risers... more on that) if I can't verify when they were replaced. I've now added heat exchangers to that list. If I can't verify they've been done in the past 7-8 years, just swap them out. I went aftermarket with Mr. Cool exchangers - they are about 1/2 the cost of OEM. I've been running them for 6+ years now without issue.
On the risers, my understanding is they are aluminum on the 8.1 motors with the dry joint system. you only need to replace the manifolds and I do them every 5 years like clock-work. You could probably milk it longer but the damage done if they fail is catastrophic so not worth the gamble in my opinion. You have 5 years runway, just plan on the expense over 5 years and it's not that bad.
Canvas - one thing that's nice about having a popular production boat is you can buy factory canvas from great lakes boat tops. They made them originally for your boat. Just give them your hull number and they can make you new ones.
Pre-covid - they were much less expensive than going to local shop to make custom canvas. I think I replaced my full canvas set for $2800. That said, the front isinglass would always start fogging and greying after 2 years. I've replaced the front glass twice now. The isinglass panels used to be $700 per set which is pretty cheap so I looked at these as 3 year throw away items.
Post-covid - great lakes and RNR just plain stopped taking orders, prices have gone up, and the wait times are long. I recently got a "camper canvas" setup custom made + replaced the front glass since it was fogging again. I finally decided I wanted to do it right and not wanting to replace the glass again for my ownership of the boat. My local canvas guy replaced the front windows and port / starboard side with poly and it is night and day in quality. They are great!!! definitely not removable or storable; the material is THICK but we never pull off the front stuff anyway.
Bedding on deck hardware is real too... I always thought i was getting excessive moisture in the cabin when we bought the boat but could never really pin point it. We're in San Diego and it doesn't rain all that often. It would rain, leak, and then most likely dry out before we got back down. One time I came down and there was water on the galley counter and bed. Took me a while to figure out but it was the deck rails that would hold the sunpad. 7 of the 8 through hull bolts were leaking, it would then travel through the headliner all over the boat.
Shaft seals are another one. Those are about a 10 year servicable lifespan. Mine started leaking right at the 10 year mark. Replaced with PIY dripless. if yours haven't been done, be ready.
That's a lot of typing and I could keep going! I have a dashboard / maintenance spreadsheet I'd be happy to share with you. I went through all the manuals for the motors, gen set, and advice from folks over the years to track when maintenance items needed to be done and when I did them last. send me your email address and I can send it over if you're interested.
The thing that concerns me the most about your boat based on the description is the 300 hours, assuming those are the original un-rebuilt motors. The worst thing you can do to a boat in my opinion is to not use it. I'd rather buy a boat with hours commensurate to regular usage based on age with a well-documented maintenance history. You wake those motors out of a 20 year slumber and start running them, you're going to start seeing all the original parts failing that normally would have been replaced with normal usage.
Bottom line for me... the 340 for has been a great boat. My broker told me at the time he sells the most of them of all Sea Ray models - it's people upgrading like me going from the 260 -> 340 and downgrading, want an easier boat to solo handle and it's crazy expensive. My kids are 6 and 3. I figure in 2-4 years, we'll "need" more space and would like to have a real stand-up shower and a stateroom. For a small family of 4 or a couple, it's a great size boat... we could even make it work with bigger kids if it doesn't make sense financially and/or we're just not using it enough to justify tripling the cost to go bigger.