Some folks leave most of the glass in place and simply cut the top off, pull the stringers out, then glass back in the new. Others cut out and remove down to the hull. The decision generally occurs based on the level of rot. If there are areas the stringers are solid, then the tabbing may still be well bonded, and cutting flush is the only option. Using the existing stringers for templates is valid, if you can get them out intact. As for tools, the vibration tool(cast cutter) will work, but a 4 or 4.5" angle grinder using cut off and standard grinding disks make the job go a bit faster, albeit dustier. Wear breathing protection, and clothing to keep the debris off your skin, as the fiberglass can really make some folks itch. As too the dash, old plastic can get brittle, possible to repair and re paint, depends on how bad the damage. There is plenty of info on this site in past threads for both issues, take a look around.