The basic motivation here is that I'm remodeling all three bathrooms because structurally, code-wise, functionally and aesthetically they are a disaster. The problems with the status quo are a litany and I don't really want to bore anyone with them, but suffice it to say that, by the time I removed the things I had to remove, it was evident that I would have to take it down to bare framing because there just wasn't enough left to try to preserve. This is mainly attributable to a previous "handyman" whose traces I'm doing my best to eradicate. In so doing, I'm hoping to "do things right" and not be another in the line of boneheaded handymen to plague what is essentially a beautifully built house.
The manner in which my house is plumbed doesn't have any venting besides the main stack, so I'm not sure that issues with a combined drain and vent are why it's not currently in use. Although I'm sure that the last remodel (by the "handyman") was in the mid-90s, by which point codes had long required this, none of the fixtures are vented. So sloth and lack of giving a hoot are a more likely explanation for the absence of any particular type of venting, and all venting in general.
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!).
One consideration that must be made is that to install fixture vents will in many cases require you to replace/reroute the drain pipes. You may have realized this but it has not been said until now.
Yes, and I'll have to replace some drain plumbing due to inappropriate fittings having been used for plane transitions, too. My hope is that, for the most part, I will be able to actually reduce the number of branches, and one of my top criteria is no new plumbing penetrations through load bearing members. I think I'll be able to accomplish all of this. It will cost a few hundred dollars more in materials and take some more time, but that's not significant compared to the overall scope of the remodel.