As far as dewinterizing, as long as your last mechanic reinstalled all the plugs, all you need to do is top off fluids and run the boat.
What engine do you have?
A lot of people from Grand Lake use Derebery Performance Marine. They can supply you with a trailer. For a supercharger, i would go with the Vortech. Derebery as a lot of boats out there with the Vortech system on them with very good results. Good luck. www.dereberyperformancemarine.com