I worked in a body shop while going to school and learned an important lesson while there. When working on any restoration project always use the least abrasive solution possible to achieve the desired result. Not saying you won't have to sand or go to 400 to get there but I would be working my way into it rather than doing the entire boat with it to start with. You'd be amazed what rubbing compound and a rotary buffer can do (good and bad). It's all going to depend on the surface condition.
I typically tackle these projects as "long term" goal type of projects and will start in one area making my decisions on an area by area basis. Restore one area, have a beer while reflecting on what went well and what might have worked better then move on to the next. If all goes well you'll be somewhat of an expert on gelcoat restoration by the time you're done.