If you wanna do it right, don't use sea cast. The reason you don't hear much about it is because most who used it used it recently and there aren't that manyy. I'm sure it works now and will last a few years, but I'm not sure it will hold up over time, it may but why take the chance and cut a corner. IMHO do it right, have the whole transom redone, either core it or use wood again but take care of it. Nothing wrong with either wood or core if you take care of it. 75% of transom problems are from owners who drilled into the transom and did not seal it properly or from thru hulls or scuppers that cracked and let water in. The rest are because the glass had air pocklets in it and lost its strength and let in water over time and fatigue. If done right it should never rot, or outlast your usage atleast. I do agree with gw204 on some of his points. There is no real way to ensure you have no voids in the hull. Fiberglass loses its strength when there is a void and becomes much easier to crack and break apart. If you leave some wood in and fill it, the wood will break down over time and those viods will crack badly. I do agree the product looks neat and in the future it might be great, but I'd be a bit leary of it honestly. It might be ok for a fix for a year or two but I would not recommend it for a lifetime fix. To redo a transom your looking at maybe $2500. Wattsdan spent $600 for the seacast plus his time to remove the wood and then fill it. Add his man hours in and the price difference is not that big.
Cutting the transom out is the easiest, remove the engine yourself if you wish to save money. You can cut it yourself too and then take it to a pro, or if you have glass expereince before you can do it yourself and then have a good yard paint the hull or gell it to match and it will look perfect and be brand new.
I know I'd never buy a boat with a seacast transom, I'd walk away in a second from the deal, no matter how good it was.
Hopefully the project goes well for you and you have no issues, I do wish you the best but personally I would not have went this route.