In my opinion, your 1st problem is what paint to use, if this boat has had bituminous paint slapped on then its not particularly good stuff, its banned in many countries because its toxic but its protection doesn't last very long, it dissolves when faced with diesel on the water and lots of other solvents it really is only OK above the water line, unfortunately the modern epoxy paints don't work too well if applied over bituminous paint either so there is a good case for removing as much as you can along the water line by hand, twisted wire cup brushes on grinders are apparently successful but I haven't tried, when the surface is as good as you can get it then a 'Surface Tolerant' epoxy like Jotun 90 (?) is the way to go. It is laborious, dirty, time consuming and just plain horrible but cheaper than blasting. As for the underneath it may never have been painted so perhaps just pressure wash and do the best you can. Jotun might even do a primer that covers bituminous paints these days but I don't know.