Just looked at the ad. That's an attractive price, so I understand your interest. I think the key will be a VERY careful inspection of the boat's condition. That price also suggests that the boat needs significant work. It could easily soak up hundreds of thousands of dollars addressing things that are just old and worn out.
I'm guessing you have a boat currently, or maybe not, but getting anything done by "professionals" is eye wateringly expensive. At $150/hr for marginally skilled techs, the meter will be spinning lightening fast with very slow progress, and figure on 10% to 50% of the work needing to be redone because it wasn't done correctly. Expect everything to be twice as expensive and take twice as long. And assume that you will only discover/identify half of the work during survey that actually needs to be done. This will be a short list of torture for a boat that appears in excellent condition, but a long list of torture for a boat that has a list of projects from the survey.
My own observation is that a fixer-upper is never a good deal if you are paying others to do the work. You will certainly have more money in the boat than if you just bought a better condition boat in the first place. Plus you will have a year or two of not using the boat, and managing refit projects.
The only way a fixer-upper works is if you are expressly looking for a project that you will mostly do yourself. Some people like projects, and others don't.
One other comment, and this isn't specific about Burr - I actually have not had any dealings with them. But I think the comments about how everything will be fine since Burr is selling the boat is just bat-**** crazy. I would never, under any circumstances, place that much blind faith in any broker or dealer no matter how much I like and/or trust them.
I hate to be a Negative Nancy, but I would proceed with caution. A $500k asking price on a Fleming 55 suggests it needs a lot of work, and is not just about its age.