Re: good bottom paint?
factory paint is a great substrate for new paint job if it's still in good shape and hasn't been covered by layers of house paint. you will need to dust* some zinc chromate or other self etching primer onto any bare areas after you lightly sand the whole hull with maybe 320 and then wash it with detergent and water,then solvent wipe once it dries and finally the primer. this is an abbreviated version of all the steps you'd take if you REALLY want to do a good job (Look at Bwana Don and his son Zach's resto of their Sea Scamp for an example of how a master does it when he really cares about doing a great job).
i should add that this is assuming you have no compatibility issues with the 2-part topcoat. this is not uncommon when applying urethane or any 2-part paint over enamel. you might want to check with the mfgr to see if you need to hit your hull with a coat of primer/sealer before topcoating to prevent any reaction betwen the topcoat and the factory enamel.
*VERY light coat, should be able to still see surface through it