Saddles weigh from 20-45lbs roughly. English saddles are lighter.
As for riding, a good rule of thumb is that a horse can carry 20-25% of it's weight. So a 1000lb horse (which is about average for a saddle horse, though arabs tend to weigh more like 900) can carry a 200 - 250lb person. If the person is an experienced rider they can generally weigh more because they know how to work with the horse and hold themselves and are less like dead weight.