It’s always a good idea to get a survey. They cost a few hundred dollars but will let you know if there are any issues major or minor. This will give you an idea of the boat’s value as well as what would be needed now, maybe needed soon, and would be nice but not required types of repairs. The buyer typically forks over the money for the survey, but sometimes the owner will split the cost as it gives him/her an idea of the vessel health as well.
If an owner will not allow a survey or a dive then I would pass on the purchase, unless you are willing to take the risk and do the work.
I purchased Lilly without a survey, but she was on a trailer and I could see the entire boat, inventory the entire set of sails/cushions and misc bits. With Luna I knew she would require an extensive amount of work and I purchased her at a marina auction. Prior to buying her I was allowed to go through everything onboard and give it my own inspection. The marina would allow a survey in the water, and a dive. I’m not a surveyor, but I am very good with mechanical repair as well as wood and glass work. So I knew what was needed on Luna was well within my skills, and I wanted a project.
So you need to decide if you have the required skills or deep enough pockets to repair anything that’s needed if you get or don’t get a survey.