The points that have been raised on here quite rightly deal with the really important stuff, if you go ahead then you have to tackle the fit out. In most, but not all, peoples opinion the best form of insulation is spray foam, if this is an elderly boat there is a chance that it is fibreglass batts or even wool. That needs to come out and the boat sprayfoamed after any welding has been done. You have to budget for, and source, a lot of plumbing, a whole lot of wiring - for which you must follow boat safety rules - some gas- again, more rules, engine installation, this can be really quite expensive and then the heating, the water tanks, great big batteries, the list goes on and on. It is not impossible, of course not, but take time over this and go and see lots of boats, I reckon the best value is a second hand, middle aged slightly tired boat with a reasonable fit out. The big projects are for those whose creative drive makes them really want to do it.