Refitting the interior makes sense IF you are handy enough to do it yourself and if you like that kind of work. If you hire it out expect it to cost tens of thousands. In a boat you can seldom change much without disturbing many other things like electrical, plumbing, etc. If you just want to change the laminates and cushions that is one thing but to do much else can add up fast. And the more you remove the more you find you want/need to change.
Marty is right, even a complete new lavac head, holding tank and associated plumbing would only set you back 2k if you do the work yourself. But installation at 80 to 100/hr will add up if you hire it done. If you are going offshore it is best you learn not only how to do this yourself but learn your own boat. This will make any repairs in the future easier to deal with.
Living aboard in Victoria Harbour