I think it depends what sort of hay you want to make. If you want rye grass seed hay plough and reseed, fertilise. You will get a lot of grass but you need someone that knows what they are doing to make it and it will take longer to dry, so haylage may be a better bet.
If you harrow, mow, over sow, fertilise, what you have it will be meadow hay and will be not so dense a crop. Its a lot more forgiving as it tends to be finer grasses and more suitable for amateur production. Finding someone to do the baling is always a problem.