@Capt Pappy
As the owner of an older boat (1976 Hatteras 48LRC) I agree that you need to check insurance and berthing first. Same on any boat, regardless of age. Just part of the boat buying process now. One marina in San Diego told me they now will not accept applications for boats older than 10 years. When space is limited you can make your own rules and still stay full. I hope this comes back to bite them but perhaps never will.
There are some jobs on old boats that are a pain but not crazy expensive, some that are easy and some that are hard AND expensive. The areas where I would really focus are:
1) Engines, the most expensive item to change out on most boats. Usually not a big problem if the boat has been reasonably maintained. Check cost and availability of parts and if there is a good mechanic in your area.
2) Hull/Decking: This can either be impractical to repair (economic) or take a lot of time (or both!). An older boat is higly likely to have some issues such as some blisters or rot in decks, If a wood core below the waterline I would be unlikely to take the chance. If decks are soft in multiple, extensive areas probably a no-go. But otherwise not huge turnoffs for me.
3) Big systems such as air-conditioning: On a boat like mine there are 4 units. I could actually replace the engines for less than this is going to cost. But the original units lasted 40 years and while I don't expect to see that service life, with the new self contained units they will be very easy and relatively inexpensive to replace.
4) Running gear: Not the most expensive items but very critical and can get into the weeds if you have to start tearing things out
5) Electronics are generally outdate the week after you get them installed. For <$10K you can get a very nice setup with GPS, Display. depth and RADAR. Be careful and only get what you need. A 16 inch screen is nice but a 10-12" tablet is likely more than enough.
The good news is most anything can be repaired or replaced and you end up with most of the functionality (perhaps almost all unless you are talking about a very advanced new boat) at a cost of perhaps 15-20% of new. And once you do all these repairs if you keep up with maintenance they can last many years or even decades.
Case in point: A Nordhavn 51 will run you about $1.5Mil and if you follow
Awanui NZ you will learn having a new boat is NOT a trouble free experience. In CA you would pay another $105K in sales tax and about $15K-$25K in property tax every year. Even a newer "lesser" boat such as a Helmsman will run $500K+. Both of these are fantastic, beautiful boats but when looking at capabilities I would argue my 1975 Hatteras 48LRC is a fair match and at a much lower cost point. Parts for my Ford Lehman engines are probably <25% of the cost of the equivalent on a new engine. Trawler speed is trawler speed so if you are OK with 7-10 knots my non-turbo, non-electronic engines are well suited and probably will run for the useful life of me and perhaps the next owner given typical coastal navigation use. Low cost to rebuild if that comes around and not too many wear parts that could be totally obsolete.
Now don't get me wrong as I love new boats too and if money were no object I might prefer a newer boat (something 3-10 years old). But I just would not enjoy boating as much (OK I could not afford it) if I had to sink $1Mil +/- into a boat. I do enjoy some of the work to keep an old boat current so that is part of my overall boating experience. Some projects I hated to do (new hoses and toilets) but I saved $5K+ and it really was not that much work. I also know how/where everything is so if maintenance is necessary it is something I can handle.
When I buy an older boat (have owned many that are well over 10-20 years old) I do a spreadsheet and itemize all the systems and a rough estimate of replacement cost of the systems. Also a list of what I think should be added (ex. LFP batteries to me are a must have if you anchor out and easily pay for themselves vs. Generator costs). I build a timeline to see what is necessary now vs down the road and figure a worse case scenario if everything became critical in a short period of time. In general I would suggest that if you buy an older boat assume in the worse case you would spend 1x the purchase price in the first 2-3 years. But of course many variables so this is an extremely broad "rule of thumb". If that number scares you then you probably should not buy an older boat. Even with a newer boat if out of warranty I think expecting 25% upcosts near term is not unreasonable. On a newer boat with a Cummins I think the intercoolers can be $5K each. Even if serviceable I think that would still be $2K+ each. So new does not equate to saving money.
At some point down the road I might be selling my boat as I recently purchased a 2011 Greenline 33. The only reason for the change is I just cannot get a 50 ft slip at our yacht club but immediately got into a 40 ft slip. For now I am keeping both boats, one in San Diego and the Hatteras in the Delta. They are totally different and I find reasons to use them both. Unlike the typical owner who just defers costs for a year or two before selling (thus the reason for the big estimated repairs/improvements) I am spending a lot to insure it is around for the next 50 years. Maybe I will own it for awhile and I want to enjoy it. I know I will never recover the costs of the work but that is OK.
I am writing this here in the boat yard after some major upfits and repairs, leaving tomorrow for my slip in the Delta. Not the least of which is 4 new ACs! It has been 5 years since I purchased Freedom. It was a very solid boat but there was a lot or room for cosmetic repairs and improvements. I assumed I would have to do the ACs right away and surprisingly all 4 made it until this year. One finally failed and while it could have been repaired it was time to bite the bullet. I got 4+ years to defer this so that was a win. Looking back I have probably spent well over 100% of the purchase price to get to where she is and that was with a LOT of sweat equity. Probably would be 150-200% if I hired it all out. Every system will have been either replaced or thoroughly renewed. Still some cosmetics to deal with but she looks good from 50 feet away and is built like a tank and very dependable. All the comforts of a new boat too at perhaps 20% of the cost.
Not sure if this is helpful but just wanted to share a bit of my perspective as a serial "old boat" owner. Good luck to you with whatever you buy!