Re: OK, so I'm Sanding My Bottom...
You don't have to re-gelcoat it. But you have to get rid of the bottom paint from the previous 3 decades or so to get any fairing material to stick to it. Apparently no previous owner bothered.
I don't think it would be very expensive - a lot of work though.
After the bottom paint is gone, fill any dips with thickened epoxy, fair with epoxy mixed with low density filler, a coat or 2 of straight epoxy and then bottom paint.
Living aboard in Victoria Harbour