Rookie in need of help

[Lottie44442]

(sorry for posting twice - not sure where is best)
Hello,
I've been researching living on a boat for months and months now and am finally at the position of buying one and am getting cold feet! It's on an auction ending in a few days! The boat is in a marina in London, off the Thames - it's an amazing location to be based and the boat is the perfect size and price for me. I'd be able to sail away my weekends, have boaty fun holidays and be close to work during the week. So far so good.

The boat is an old 1970s Westerly, GRP hull. I've gotten the impression that it's previous owners didn't use it that much and don't know when it was last out of the water - should alarm bells be ringing? I've researched and researched but can't find a definitive answer - is it terribly bad to leave a boat in a marina in the water like this? Is the Thames be a much of a corrosive enviroment for it?

The description of the boat as is that it has 'many more miles left in her' which is meant to be a positive, but make me feel negative.. like she has a finite lifespan and will pack up in a number of years.
I know that nothing lasts forever but I don't want to spend a large amount of money and time on something which is end up in the scrap heap - I want something solid which will stay in the family for decades.... is that possible or am I asking too much of an old boat?

I will get a full survey done, but it would be so so helpful to get some insight from more experienced boaters. (I've only sailed them and not bought one before!)

Thank you so much!
Lottie