firstly Stuart it depends on how far your budget will stretch. Perhaps it would go further with the sailaway as you would have more time to do it.
Obviously you will have to assess your own diy competence as well, it is a multi-skilled job.
Don't underestimate the time it will take to fit out a sailaway (or for that matter to rip out & refit an old hull, but you will finish up with a boat with much higher value or one htat will last you happily 20+ years.
Graham booth's book on fitting out would be indispensible, and could also give you an idea of total cost.
Then there is what type of sailaway: ordinary, lined or lined plus. The last of these would be fully lined with electrics and possibly some plumbing partly done, but obviously the more you have done the more it will cost.
I'm currently giving serious thought to getting a lined sailaway for my next boat. If you keep it for 5 years you get round the RCD problems, and it is not unusual to come across "owner fitouts" so it will still be saleable, providing the work is done to a reasonable standard.
An old boat will always be an old boat, even if you have stripped it out and refitted it. A sailaway starts as a new boat, and will you really save that much if you are going to strip out most of the interior of an old boat?
Don't underestimate the amount of time it will take though. I'm reliably informed that, assuming you have a full-time job, three years is the typical time it takes to fully complete the project.