The size of the main vent depends on how many fixtures you connect and where you live (snow, cold country often requires a larger section before and through the roof to prevent it from closing due to hoar frost). You can combine vents to minimize roof penetrations if you wish, but it is not required if you can live with multiple roof penetrations. There are specific rules on how and where you can combine vents.
It was quite common to use S-traps, and wet vent things a long time ago, and things generally worked most of the time. There are always exceptions, and remodeling can create some of those situations where they don't work well anymore, so doing it right now when you have the chance is good thinking! (and will make your plumbing inspector appeased, as well!).