You might luck out using the old gaskets, but it is not a good bet.
If you want to try to save some money, go with the old gaskets. It's your time wasted if it they do not seal. Sealer is not normally used or required, but, with old gaskets, you might want to use a high-temperature gasket sealer on both sides of the old gaskets.
Before installing the new assembly, place it on a flat surface to see if the mating surface is flat from end-to-end.
