Well maintained older boats will already have components or systems replaced or upgraded as needed. Obviously it depends on the particular boat, and you can't make sweeping generalizations about them.
From the sound of your question, and the fact that you would consider anything in the spectrum from the cheap French production boats, to blue water cruisers, to Swans, I suspect you haven't decided upon the type of sailing you expect to do, or what you expect from the boat. If you're serious about sailing, I'd take a couple years to sort out these issues before buying a 50-footer. On the other hand, if you just want something that looks nice for dockside parties and bragging rights, then jump right in there and spend as much as your ego needs.